The Mordern Bajaj Chetak Rides One
This is a procedure that you should read entirely before lifting the first wrench.  To replace the lever you will need to deal with connections at the lever end and at the engine end, so it pays to know in advance what you will encounter.

To replace the clutch lever, you need to remove the single nut from the bottom of the headset on the clutch lever side. Then, using a flat-head screwdriver, unscrew the threaded bolt from the bottom of the headset. You now have to disconnect the cable from the lever. You will need to get down by the enigne and loosen the pinch bolt so you can slide enough cable up to give you spare cable at the lever to disconnect the cable from the lever. Then, be careful as you remove the lever as there are two small washers that will come out with it - one on top and one on the bottom. Save these, as you'll need them! 

This is probably also a good time to replace the cable.  Inspect the cable outer to make sure it's in good shape. Lubricate the new clutch cable using a lube of your preference - grease or chainsaw chain oil  etc (each has proponents and detractors). Thread in your new clutch cable from the headset. Don't connect the cable to anything just yet. 

Now install your new clutch lever. It can be tricky with those little washers, but be persistent and you'll get it. Grease up the clutch lever on both sides and in the bolt hole. Put a washer on each side of the clutch lever and insert the threaded bolt through the bottom of the headset, through the washers and clutch lever, and into the threaded part of the headset. Be very careful with threading the bolt in because it is super easy to strip the threads in the headset (which is a difficult repair). Once you've got the bolt threaded in correctly, thread on the nut and tighten it to the point where it is nice and snug but you can still operate the clutch lever with very little resistance. 

Now put the clutch cable into the retaining hole on the clutch lever. Grease this area well as it sees a lot of movement which causes parts to rub against each other and wear. Pull on the clutch lever from the engine side of the frame and make sure things are working well. Thread the cable properly through the adjuster and clutch actuator arm on the engine side. Attach the pinch bolt behind the clutch actuator arm and adjust the clutch cable to your preference. 

The barrel adjuster is only for taking up a limited amount of cable slack. When you run out of adjuster room, turn the barrel adjuster back down to minimum thread, but do not bottom it out. Your clutch cable should now have some slack that needs to be taken out:

Look for the clutch lever on the bottom of the engine. The clutch cable goes to this lever and a bolt type cable clamp. To adjust, you loosen the bolt on the clamp (the right size wrench is in the tool kit that came with the scooter). grab the cable with some pliers and pull the slack out of it, then push the clamp up against the lever. Don't push the lever itself otherwise you will partly disengage the clutch when it is supposed to be fully engaged. Using your own third hand, a third hand tool, or a friend's hand, tighten the bolt. The clutch should be adjusted at this point. Now try riding. If you get that whining sound, and the gear change is more notchy than normal with the lever all the way out, then there is still too much slack. 
If clutch cable adjuster bolt/assembly snaps off

It's a right-hand thread, meaning that if you turn is counter-clockwise
(from either end) it will come out.
If there is a portion of the hollow tube extended beyond the casting, you
could saw a slot to accommodate a screwdriver. If you have an "easy out"
that would be perfect for this job, unfortunately most people don't have

Soaking the part first with some penetrating oil and heating the aluminum
casting will help. Do this several times before trying to remove it. If one
end has damaged thread, you do not want to screw these damage thread through
the body, they will damage the internal threads in the aluminum casting.
Because of the hole through the bolt, this part is more fragile than in
appears. When you finally replace the adjuster, don't make the lock nut too

The Modern Bajaj Chetak Rides On The Modern Bajaj Chetak Rides On