Onesie's bound to fit (8)

I believe the answer is:

jumpsuit

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'onesie's' is the definition.
I can't judge whether this defines the answer.

'bound to fit' is the wordplay.
'bound' becomes 'jump' (I've seen this in another clue).
'to fit' becomes 'suit' ('suit' can be a synonym of 'fit').
'jump'+'suit'='JUMPSUIT'

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(Another definition for jumpsuit that I've seen is " One-piece garment".)

I've seen this clue in The Guardian.

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