Pickled herring fillet (7)
I believe the answer is:
rollmop
'pickled herring fillet' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rollmop that I've seen before include "Dish of herring fillet" , "Curled-up herring fillet" , "Rolled and pickled herring" , "Herring pickled in spiced vinegar" , "savoury dish" .)