If you want fish, crack a nut! (4)

I believe the answer is:

tuna

I'm a little stuck... Click here to teach me more about this clue!

'if you want fish' is the definition.
I can't judge whether this defines the answer.

'crack a nut' is the wordplay.
'crack' is an anagram indicator.
'a'+'nut'='anut'
'anut' is an anagram of 'TUNA'.

Can you help me to learn more?

(Other definitions for tuna that I've seen before include "Marine food for your strange aunt" , "Marine food fish" , "Popular food fish" , "Tunny" , "Fish sold in cans" .)

I've seen this clue in the Evening Standard.

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