My, that is something only the young should wear (6)
I believe the answer is:
onesie
'something only the young should wear' is the definition.
I don't know anything 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' (abbreviation for id est, 'that is' in Latin).
'ones'+'ie'='ONESIE'
Can you help me to learn more?
(Other definitions for onesie that I've seen before include "One-piece full-body garment" , "Single-piece full body garment" .)