A dry male getting large one quickly (2,3,6)
I believe the answer is:
at the double
'quickly' is the definition.
(I've seen this before)
'a dry male getting large one' is the wordplay.
'dry' becomes 'tt' (abbreviation for teetotal).
'male' becomes 'he'.
'getting' says to put letters next to each other.
'large one' becomes 'double' (a large drink).
'a'+'tt'+'he'+'double'='AT THE DOUBLE'
(Other definitions for at the double that I've seen before include "Where arrow is aimed, initially" , "Pronto" , "Sharpish" , "in half the usual time?" , "Without delay" .)