British subject about to punch drunk (4)
I believe the answer is:
scot
'british subject' is the definition.
(someone from Scotland)
'about to punch drunk' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'to punch' means one lot of letters goes inside another.
'drunk' becomes 'sot' (both can mean someone who drinks too much).
'c' put inside 'sot' is 'SCOT'.
(Other definitions for scot that I've seen before include "Native of Glasgow, say" , "Aberdonian possibly" , "levy, once" , "One from northern UK" , "Northerner; old tax" .)