Fish product that's substandard turned up containing still more in the middle (7)
I believe the answer is:
rollmop
'fish product' is the definition.
'rollmop' can be an answer for 'fish' (type of herring fillet). I'm not sure about the 'product' bit.
'substandard turned up containing still more in the middle' is the wordplay.
'substandard' becomes 'poor'.
'turned up' says the letters should be written in reverse.
'containing still' becomes 'll' (I can't justify this - if you can you should give a lot more credence to this answer).
'more' becomes 'm'.
'in the middle' indicates putting letters inside.
'poor' in reverse letter order is 'roop'.
'll'+'m'='llm'
'roop' enclosing 'llm' is 'ROLLMOP'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for rollmop that I've seen before include "Herring pickled in spiced vinegar" , "Fish" , "Pickled fillet of raw herring" , "Food" , "Curled-up herring fillet" .)