Fix up a drink (3)
I believe the answer is:
nip
'a drink' is the definition.
(I've seen this before)
'fix up' is the wordplay.
'fix' becomes 'pin' (pinning is a kind of fixing).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'pin' written backwards gives 'NIP'.
(Other definitions for nip that I've seen before include "(Give) a small sharp bite" , "Pinch sharply, perhaps in the bud" , "severe cold" , "Pinch or squeeze sharply" , "small draught" .)