Herring recipe with cool clam soup periodically used (7)
I believe the answer is:
rollmop
'herring' is the definition.
(I've seen this before)
'recipe with cool clam soup periodically used' is the wordplay.
'recipe' becomes 'r' (abbreviation in medical prescriptions).
'with' is a charade indicator (letters next to each other).
'periodically used' means one should take alternating letters (letters selected regularly or periodically).
'cool'+'clam'+'soup'='coolclamsoup'
The alternate letters of 'coolclamsoup' are 'ollmop'.
'r'+'ollmop'='ROLLMOP'
(Other definitions for rollmop that I've seen before include "savoury dish" , "Curled-up herring fillet" , "swimmer that's soused" , "Pickled herring wrapped around onion slices" , "Dish of herring fillet" .)