In the workshop manual there's a laundry list of things that it can be..
That said my bike is an 08 zx6r and I'm not familiar with the exact differences in our bikes..
I can tell you that I've had this problem more than once and it was from totally different reasons both times..
"Contaminated fuel" was the root cause this last round of troubleshooting..
The manual had a list of No or Low power in low revs , clutch not properly disengaging was on the list as I remember checking things connected to that..
But the most common reasons this seems to happen are
-Carbs or Injectors need cleaning
-Fuel pump needs replacing or
-Fuel filter/Fuel strainer needs cleaning or replacement.
-Electronic component failure (ECU/CDI/Battery etc)
- Electrical short or bad connections
I'm not saying it couldn't be the clutch.. but its extremely frustrating to go spend a whack of cash on something (even if it needs to be done sooner or later)
Jumping on your bike all excited and hoping that the problem is gone...and ....nothing.. zero...dead stick...problem is still there
Start with the simple things to check..
Grab a multimeter and start there
..have you got the workshop manual for your bike?
Likewise bud.. my fuel pump I've stripped out and cleaned now a good few times (dodgy tank I bought had a split in the metal and was bleeding crap into my fuel)
It's a good thing to clean out anyhow..
Just be careful not to prime your fuel pump when it's not submerged in liquid..
If your bike uses the same T35 fuel pump and fuel assembly configuration as many of the other kawi's do then its a relatively simple job to do.. there are a few good guides on YouTube and a useful trick is to take a LOT of pictures while you're busy , at each step .. this will save you a TON of hassles if/when you run into a snag or something...