Friends about to deliver blow (4)
I believe the answer is:
slap
'blow' is the definition.
(I know that slap is a type of blow)
'friends about' is the wordplay.
'friends' becomes 'pals' (pal is a kind of friend).
'about' says the letters should be written backwards.
'pals' back-to-front is 'SLAP'.
'to deliver' is the link.
(Other definitions for slap that I've seen before include "Sharp smack" , "Right" , "Hit with an open hand" , "Blow (in face, on back, wrist)" , "Mild punishment" .)