If the slot is closed up then perhaps you should. Has the lever been previously re-broached so that it is over size?
Mine was the same - I carefully went down the slot with a cutting wheel on an angle grinder and it's fine now. If you slip a bit of sheet steel down the slot, put the bolt in the threaded side then tighten against it, you can open it out safely for better access.
Check out the shaft before you do anything. Could be that`s come to the end of its life.I am thinking of "shaving" off a small amount with the slot in its relaxed position with an abrasive disk on my dremel tool. This should remove less metal than the hacksaw blade. I may see if feeler gauges tell me where the metal needs to be removed, if the slot is tapered while clamped.
The bolt is very tight, but the lever still rocks a bit on the shaft. To protect the serrations, should I widen the clamp slot?