Man on board catching river shellfish (5)
I believe the answer is:
prawn
'shellfish' is the definition.
(thesaurus)
'man on board catching river' is the wordplay.
'man on board' becomes 'pawn' (chess).
'catching' indicates putting letters inside.
'river' becomes 'r'.
'pawn' enclosing 'r' is 'PRAWN'.
(Other definitions for prawn that I've seen before include "Cocktail ingredient" , "Popular oriental snack" , "basis for cocktail" , "Large shrimp" , "Marine crustacean" .)