Seafood dish, one served with pickle (6)
I believe the answer is:
scampi
'seafood dish' is the definition.
(I've seen this before)
'one served with pickle' is the wordplay.
'one' becomes 'i' (Roman numeral).
'served with' means one lot of letters go next to another.
'pickle' becomes 'scamp' (I've seen this before).
'i' put after 'scamp' is 'SCAMPI'.
(Other definitions for scampi that I've seen before include "Dish of crustaceans" , "Prawns (fried with breadcrumbs)" , "Large prawns cooked in batter" , "Large prawns as food" , "Crumby seafood dish" .)