This has nothing on marine fish producer? (4)
I believe the answer is:
farm
'fish producer?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'this has nothing on marine' is the wordplay.
I cannot really understand how this works, but
'nothing' could be 'fa' (slang term - sweet Fanny Adams) and 'fa' is found in the answer.
'marine' could be 'rm' and 'rm' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for farm that I've seen before include "and 27: Source of meat" , "agricultural centre" , "Ranch" , "Grow crops or livestock" , "croft" .)