British monarch keeps bird in hat (6)
I believe the answer is:
bowler
'hat' is the definition.
(bowler is a kind of hat)
'british monarch keeps bird' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'keeps' is an insertion indicator.
'bird' becomes 'owl' (owl is a kind of bird**).
'b'+'er'='ber'
'ber' going around 'owl' is 'BOWLER'.
'in' is the link.
(Other definitions for bowler that I've seen before include "Round black hat with brim" , "Stiff hat with rounded crown" , "something more substantial than a cap" , "See 8" , "Posh hat or cricket player" .)