Without water swallow single drink (3,7)
I believe the answer is:
dry martini
'drink' is the definition.
(I've seen this before)
'without water swallow single' is the wordplay.
'without water' becomes 'dry'.
'swallow' becomes 'martin' (martin is a kind of swallow**).
'single' becomes 'i' (one).
'dry'+'martin'+'i'='DRY MARTINI'
(Other definitions for dry martini that I've seen before include "Gin-based cocktail" , "drink" .)