A young fish becomes crook inside a sock (7)

I believe the answer is:

sillock

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'a young fish' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'crook inside a sock' is the wordplay.
'crook' becomes 'ill' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'inside' means one lot of letters goes inside another.
'ill' put within 'sock' is 'SILLOCK'.

'becomes' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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