Man on board eating right seafood (5)
I believe the answer is:
prawn
'seafood' is the definition.
(prawn is a kind of seafood)
'man on board eating right' is the wordplay.
'man on board' becomes 'pawn' (chess).
'eating' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'pawn' enclosing 'r' is 'PRAWN'.
(Other definitions for prawn that I've seen before include "Small, edible crustacean" , "Shrimp-like crustacean" , "basis for cocktail" , "Shellfish" , "Small tasty crustacean, nice in cocktail" .)