First class fish and pork (9)
I believe the answer is:
crackling
'first class' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'fish and pork' is the wordplay.
'fish' becomes 'ling' (ling is a kind of fish).
'and' means one lot of letters go next to another.
'pork' becomes 'crack' (I can't justify this - if you can you should give a lot more credence to this answer).
'ling' put after 'crack' is 'CRACKLING'.
Can you help me to learn more?
(Other definitions for crackling that I've seen before include "Pork covering" , "Crisp skin of pork joint" , "Crisp exterior of roast pork" .)