Clothing in which one does not puff so much! (10)
I believe the answer is:
underpants
'clothing' is the definition.
(underpants is a kind of clothing)
'one does not puff so much' is the wordplay.
I cannot quite see how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is present in the answer.
'not' could be 'nt' (synonyms) and 'nt' is found in the answer.
'so' could be 'as' (e.g. phrases like 'not as tall' and 'not so tall') and 'as' is present in the remaining letters.
This explanation may well be incorrect...
'in which' acts as a link.
Can you help me to learn more?
(Other definitions for underpants that I've seen before include "Shorts" , "Close-fitting garment" , "Intimate clothing" , "cover and support" , "drawers" .)