British traitor is a rascal (4)
I believe the answer is:
brat
'rascal' is the definition.
(brat is a kind of rascal)
'british traitor' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'traitor' becomes 'rat' (rat can mean a treacherous person).
'b'+'rat'='BRAT'
'is a' is the link.
(Other definitions for brat that I've seen before include "Unpleasant youngster" , "Ill-mannered child" , "child - that's a nuisance" , "A spoiled, badly behaved child" , "Irritating youngster" .)