Left atmosphere in hideout (4)
I believe the answer is:
lair
'hideout' is the definition.
(I've seen this in another clue)
'left atmosphere' is the wordplay.
'left' becomes 'l' (common abbreviation).
'atmosphere' becomes 'air' (synonyms).
'l'+'air'='LAIR'
'in' acts as a link.
(Other definitions for lair that I've seen before include "Private place of seclusion" , "Rail around animal's den" , "enclosure" , "Wild animal's den" , "Earth" .)