Fish in action worked out (7)

I believe the answer is:

decoded

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'worked out' is the definition.
The answer and definition can be both related to thinking as well as being past participle verbs.
Maybe there's a link between them I don't understand?

'fish in action' is the wordplay.
'fish' becomes 'cod' (type of fish).
'in' indicates putting letters inside.
'action' becomes 'deed' (deed is a kind of action).
'cod' placed inside 'deed' is 'DECODED'.

Can you help me to learn more?

I've seen this clue in The Sun.

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