Mopers hate fresh air (10)
I believe the answer is:
atmosphere
'air' is the definition.
(I know that air can be written as atmosphere)
'mopers hate fresh' is the wordplay.
'fresh' indicates an anagram (a fresh re-ordering of the letters).
'mopers'+'hate'='mopershate'
'mopershate' anagrammed gives 'ATMOSPHERE'.
(Other definitions for atmosphere that I've seen before include "Feeling" , "climate" , "Gas surrounding a planet" , "blue?" , "Spirit" .)