Seafood failed to win over British monarch (7)
I believe the answer is:
lobster
'seafood' is the definition.
(lobster is a kind of seafood)
'failed to win over british monarch' is the wordplay.
'failed to win' becomes 'lost' ('lose' can be a synonym of 'fail to win').
'over' means one lot of letters goes inside another.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'lost' going around 'b' is 'lobst'.
'lobst'+'er'='LOBSTER'
(Other definitions for lobster that I've seen before include "Large marine crustacean" , "Bolster up te shellfish" , "Slow bowler may be" , "Bolster up the sea-creature" , "one to loft a > shellfish?" .)