Clutch question

I've got an 89 Civic 4WD wagon with a manual tranmission. If I'm cruising at a constant speed, say 40 mph, and then push the gas pedal down, the engine will rev up before the car actually accelerates, as if I was stepping on the clutch at the same time. It only does it under hard acceleration. Other than that, it seems OK. What is happening?

sounds like clutch is slipping.

have you tried adjusting the clutch cable ?

No, not yet. I've never done that. Can you easily tell me how to check it?

pull up on the clutch cable, at the clutch arm turn the knob and tighten it till there is very little play in the arm

or put a new clutch in