Pickled herring (7)
I believe the answer is:
rollmop
'pickled herring' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rollmop that I've seen before include "Ex-swimmer preserved" , "Pickled fillet of raw herring" , "swimmer that's soused" , "savoury dish" , "Curled-up herring fillet" .)