Agree with boy eating fish (5,2)

I believe the answer is:

shake on

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'agree' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'boy eating fish' is the wordplay.
'boy' becomes 'son' (synonyms).
'eating' is an insertion indicator.
'fish' becomes 'hake' (hake is a kind of fish).
'son' going around 'hake' is 'SHAKE ON'.

'with' acts as a link.

Can you help me to learn more?

I've seen this clue in The Guardian.

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