Pickled fillet of raw herring (7)
I believe the answer is:
rollmop
'pickled fillet of raw herring' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rollmop that I've seen before include "seafood?" , "swimmer that's soused" , "Ex-swimmer preserved" , "Dish of herring fillet" , "Pickled herring wrapped around onion slices" .)