Airless (6)
'airless' is the definition.
(I know that airless can be written as stuffy)
This is all the clue.
(Other definitions for stuffy that I've seen before include "needing to be aired" , "Staid - airless" , "Airless - humourless" , "Airless; unadventurous" , "Old-fashioned" .)
(Other definitions for frowsty that I've seen before include "stuffy" , "Foul-smelling" .)