Be under the delusion that one's only a child, too (3,7)

I believe the answer is:

kid oneself

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'be under the delusion that one's only a' is the definition.
I know nothing about this answer so I can't judge whether this works.

'child too' is the wordplay.
'child' becomes 'kid' (synonyms).
'too' becomes 'oneself' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'kid'+'oneself'='KID ONESELF'

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I've seen this clue in the Evening Standard.

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