Blow as friends turned up (4)
I believe the answer is:
'blow' is the definition.
(I know that slap is a type of blow)
'friends turned up' is the wordplay.
'friends' becomes 'pals' (pal is a kind of friend).
'turned up' says the letters should be written in reverse.
'pals' back-to-front is 'SLAP'.
'as' is the link.
(Other definitions for slap that I've seen before include "Make-up (slang)" , "Hit with the palm" , "Strike with the palm" , "Blow, maybe in the face" , "Cuff" .)