Ruin atmosphere in retreat (4)
I believe the answer is:
doom
'ruin' is the definition.
(thesaurus)
'atmosphere in retreat' is the wordplay.
'atmosphere' becomes 'mood' (I've seen this before).
'in retreat' is a reversal indicator.
'mood' back-to-front is 'DOOM'.
(Other definitions for doom that I've seen before include "Mood of death or ruin" , "Destiny; ruin" , "In disaster" , "Mood for damnation" , "Mood of grim fate" .)