Partner's drink returned (3)
I believe the answer is:
pal
'partner's' is the definition.
(pal can mean a friend)
'drink returned' is the wordplay.
'drink' becomes 'lap' (to lap is to drink up).
'returned' says the letters should be written in reverse.
'lap' written backwards gives 'PAL'.
(Other definitions for pal that I've seen before include "Bud" , "Mate, informal friend" , "Crony" , "Buddy, chum" , "Brother" .)