Shellfish at sea? British tucking in with hesitation (7)
I believe the answer is:
lobster
'shellfish' is the definition.
(I know that lobster is a type of shellfish)
'at sea? british tucking in with hesitation' is the wordplay.
'at sea?' becomes 'lost' (at sea can mean in lost or in confusion).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'tucking in' means one lot of letters goes inside another.
'with' means one lot of letters go next to another.
'hesitation' becomes 'er' (sound used when hesitating).
'lost' enclosing 'b' is 'lobst'.
'lobst'+'er'='LOBSTER'
(Other definitions for lobster that I've seen before include "Crustacean with pincer-like claws" , "Slow bowler may be" , "Bolster (anag.)" , "Large, strong-jawed crustacean" , "Marine creature" .)