Blow friends up (4)
I believe the answer is:
slap
'blow' is the definition.
(slap is a kind of blow)
'friends up' is the wordplay.
'friends' becomes 'pals' (pal is a kind of friend).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'pals' backwards is 'SLAP'.
(Other definitions for slap that I've seen before include "Blow (in face, on back, wrist)" , "Strike with the palm" , "Strike action" , "Blow with open hand" , "Hit with an open hand" .)