Fish stick bought at seaside (4)

I believe the answer is:

rock

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'fish' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe you can see an association between them that I can't see?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for rock that I've seen before include "Sweet < music" , "Brighton - (novel); - Hudson" , "lull baby to sleep?" , "Style of music; shock" , "Shake; boulder" .)

I've seen this clue in The Guardian.

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