Open, it's not stuffy (7)
I believe the answer is:
unclose
'open' is the definition.
(I've seen this before)
'not stuffy' is the wordplay.
'not' becomes 'un' (I've seen this in other clues).
'stuffy' becomes 'close' ('close' can be a synonym of 'stuffy').
'un'+'close'='UNCLOSE'
'it's' acts as a link.
(Other definitions for unclose that I've seen before include "Open" , "put on view" .)