Mates turned up in make-up (4)
I believe the answer is:
slap
'make-up' is the definition.
(informal term for make-up)
'mates turned up' is the wordplay.
'mates' becomes 'pals'.
'turned up' shows that the letters should be reversed in order.
'pals' reversed gives 'SLAP'.
'in' acts as a link.
(Other definitions for slap that I've seen before include "Hit with a flat hand" , "Sharp smack" , "Hit with the palm" , "Right" , "Hit with an open hand" .)