Instantly fancy a go (4,1,4)
I believe the answer is:
like a shot
'instantly' is the definition.
(I've seen this before)
'fancy a go' is the wordplay.
'fancy' becomes 'like' (I've seen this before).
'go' becomes 'shot' (I've seen this before).
'like'+'a'+'shot'='LIKE A SHOT'
(Other definitions for like a shot that I've seen before include "straight away" , "sluggish?" , "Fancy a drop to drink" , "What tequila drinkers might do" , "Outright" .)