This behind pupil swallowing second drink (7)
I believe the answer is:
retsina
'drink' is the definition.
(retsina is a kind of drink)
'this behind pupil swallowing second' is the wordplay.
'this behind pupil' becomes 'retina'.
'swallowing' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'retina' enclosing 's' is 'RETSINA'.
(Other definitions for retsina that I've seen before include "S European wine" , "Greek wine - nastier sort" , "Nastier (anag) - white (or rose) wine" , "Greek drink" , "Oily-tasting Greek white wine" .)