Man on board catches right shellfish (5)
I believe the answer is:
prawn
'shellfish' is the definition.
(I've seen this before)
'man on board catches right' is the wordplay.
'man on board' becomes 'pawn' (chess).
'catches' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'pawn' enclosing 'r' is 'PRAWN'.
(Other definitions for prawn that I've seen before include "Cocktail ingredient" , "Cocktail item" , "Large shrimp" , "Small tasty crustacean, nice in cocktail" , "Bit of seafood" .)