Little drink of pop (3)
I believe the answer is:
nip
I believe this clue is a double definition.
'little drink' is the first definition.
(I've seen this before)
'pop' is the second definition.
('nip out' and 'pop out' can both mean to go out)
'of' is the link.
(Other definitions for nip that I've seen before include "Pinch; quantity of spirits" , "Pinch - tot" , "Swift bite" , "small draught" , "Pinch sharply, perhaps in the bud" .)