Friend having drink knocked back (3)
I believe the answer is:
pal
'friend' is the definition.
(pal can mean a friend)
'drink knocked back' is the wordplay.
'drink' becomes 'lap' (to lap is to drink up).
'knocked back' is a reversal indicator.
'lap' backwards is 'PAL'.
'having' acts as a link.
(Other definitions for pal that I've seen before include "China" , "Close friend" , "Crony" , "Mate, buddy" , "Brother" .)