Childish brainstorm? (5)

I believe the answer is:

bairn

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'childish brainstorm?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

This is the entire clue.

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(Other definitions for bairn that I've seen before include "Kid" , "Child in Scotland" , "Scot. child" , "Wee child" , "Scots child" .)

I've seen this clue in the Evening Standard.

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