How can you be sure there is no sign of fuel starvation? In 1st and 2nd gear the engine RPM rises very quickly and so in these two gears the apatite of the engine is greatest for fuel. Any starvation symptoms will be most apparent here. I am assuming your bike has a carburettor. What may be happening is that the float bowl is not getting filled up at the same rate as the engine is consuming fuel. Or the needle is set too low (to save fuel) and at full throttle, the fuel/air mixture is too lean to give you the power you are looking for. Both cases can be classified as fuel starvation.
The test for slipping clutch is as follows:
Run your bike on a quiet road (with little traffic). Proceed at a steady speed of say 50km/h in the highest gear (4th or 5th). Slowly and progressively apply the REAR brake, your bike will begin to slow down, compensate the bike's slowing down by opening the throttle progressively. If your clutch is slipping, you will note the engine RPM is steady or increases while the bike slows down. If the clutch is good, the engine will stall because of the braking action. During this test do not depress the clutch at any time.
BTw what bike are we talking about?