I have an 03 chevy blazer. When I turn on the ac the compressor sometimes kicks on sometimes it won't. But I can go under the hood and touch the front of the compressor and it will kick on every time but it cut off after I start driving. I was told it may be a bad clutch if it is can that be replaces without replacing the compressor... help!

Yes, the compressor clutch and hub can be replaced without replacing the entire unit.
This saves time but also saves money because the system doesn't need discharged and then recharged.
If the compressor is operating quietly and cooling properly, I would replace the clutch and hub only.