Fishy way to turn a nut (4)
I believe the answer is:
tuna
'fishy way' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'turn a nut' is the wordplay.
'turn' says the letters should be written in reverse.
'a'+'nut'='anut'
'anut' back-to-front is 'TUNA'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for tuna that I've seen before include "Large edible sea fish, often canned" , "swimmer" , "Marine food fish" , "Large sea-fish" , "Large food fish" .)