Pickled herring to start followed by sponge (7)
I believe the answer is:
rollmop
'pickled herring' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for rollmop that I've seen before include "Pickled herring on a stick" , "Ex-swimmer preserved" , "Curled-up herring fillet" , "Herring pickled in spiced vinegar" , "Fish" .)