Bears seen in British castles (6)
I believe the answer is:
brooks
'bears' is the definition.
('brook' can be a synonym of 'bear')
'british castles' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'castles' becomes 'rooks' ('rook' can be a synonym of 'castle').
'b'+'rooks'='BROOKS'
'seen in' acts as a link.
(Other definitions for brooks that I've seen before include "Countenances" , "Former editor" , "Small streams of water" .)