Drinks a Spitfire becoming squiffy (9)
I believe the answer is:
aperitifs
'drinks' is the definition.
(aperitif is a kind of drink)
'a spitfire becoming squiffy' is the wordplay.
'becoming squiffy' is an anagram indicator (squiffy can mean drunk or wonky).
'spitfire' anagrammed gives 'peritifs'.
'a'+'peritifs'='APERITIFS'
(Other definitions for aperitifs that I've seen before include "Starters" , "Pre-dinner drinks" .)