Fancy a whisky? Very quickly! (4,1,4)
I believe the answer is:
like a shot
'very quickly' is the definition.
(I've seen this in another clue)
'fancy a whisky?' is the wordplay.
'fancy' becomes 'like' (both can mean to find someone attractive).
'whisky?' becomes 'shot' (shot of whisky).
'like'+'a'+'shot'='LIKE A SHOT'
(Other definitions for like a shot that I've seen before include "looking round, in field event?" , "At once, willingly" , "Lout" , "Outright" , "Fancy a drop to drink" .)