Less fresh challenger at first one defeated (6)
I believe the answer is:
closer
'less fresh' is the definition.
(close can mean humid or stuffy)
'challenger at first one defeated' is the wordplay.
'at first' says to take the initial letters.
'one defeated' becomes 'loser'.
The first letter of 'challenger' is 'c'.
'c'+'loser'='CLOSER'
(Other definitions for closer that I've seen before include "Nearer or more intimate" , "Stuffier" , "denser" , "Nearer in time or space" , "Comparatively near" .)