My, that is something only the young should wear (6)

I believe the answer is:

onesie

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'something only the young should wear' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'my that is' is the wordplay.
'my' becomes 'ones' ('one's' is a humorous equivalent to 'my').
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'ones'+'ie'='ONESIE'

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(Other definitions for onesie that I've seen before include "One-piece full-body garment" , "Single-piece full body garment" .)

I've seen this clue in The Independent.

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