Revolting friend's make-up (4)
I believe the answer is:
slap
'make-up' is the definition.
(informal term for make-up)
'revolting friend's' is the wordplay.
'revolting' is a reversal indicator.
'friend's' becomes 'pal's' (pal can mean a friend).
'pals' in reverse letter order is 'SLAP'.
(Other definitions for slap that I've seen before include "Right" , "Strike action" , "Cuff" , "Strike (in the face)" , "Mild punishment" .)