One in position for seafood (6)
I believe the answer is:
plaice
'for seafood' is the definition.
(plaice is a kind of seafood)
'one in position' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'position' becomes 'place' (place is a kind of position).
'i' put into 'place' is 'PLAICE'.
(Other definitions for plaice that I've seen before include "Edible flatfish" , "Large flat fish" , "Location of the fish" , "Type of fish" , "Flat fare" .)