Stuffiest bar on street (7)
I believe the answer is:
closest
'stuffiest' is the definition.
('close' can be a synonym of 'stuffy')
'bar on street' is the wordplay.
'bar' becomes 'close' (bar can mean to close or obstruct).
'on' is a charade indicator (letters next to each other).
'street' becomes 'st' (abbreviation).
'close'+'st'='CLOSEST'
(Other definitions for closest that I've seen before include "Nearest or meanest" , "Most intimate" , "Most secretive" , "Superlatively dense" , "Nearest or most sultry" .)