Pickled fillet of raw herring (7)
I believe the answer is:
rollmop
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">pickled fillet of raw herring</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)