British traitor is a whippersnapper (4)
I believe the answer is:
brat
'a whippersnapper' is the definition.
(I've seen this in another clue)
'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 "Urchin" , "child - that's a nuisance" , "Bad child - pack member!" , "Mervyn -- (Titus Groan)" , "Little terror" .)