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 see how one could define the 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 am not sure about this - if you are sure you should believe this answer much more).
'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" .)