United side lacking depth — it looks silly to some (6)
I believe the answer is:
onesie
'it looks silly to some' is the definition.
I know nothing about this answer so I can't judge whether this works.
'united side lacking depth' is the wordplay.
'united' becomes 'one' (as in a phrase like 'we are one').
'lacking' indicates named letters should be taken away.
'depth' becomes 'd' (abbreviation in geometry).
'side' with 'd' removed is 'sie'.
'one'+'sie'='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" .)