Friend's revolting make-up (4)
I believe the answer is:
'make-up' is the definition.
(informal term for make-up)
'friend's revolting' is the wordplay.
'friend's' becomes 'pal's' (pal can mean a friend).
'revolting' is a reversal indicator.
'pals' reversed gives 'SLAP'.
(Other definitions for slap that I've seen before include "Strike (in the face)" , "Blow, maybe in the face" , "A blow among pals" , "Make-up (slang)" , "Hit with an open hand" .)