British traitor is an urchin (4)
I believe the answer is:
brat
'an urchin' is the definition.
(both can mean a child)
'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' acts as a link.
(Other definitions for brat that I've seen before include "child - that's a nuisance" , "Little terror" , "Marie ---, birth-control pioneer" , "Annoying child" , "Bad child - pack member!" .)