Atmosphere associated with large animal’s retreat (4)
I believe the answer is:
lair
'animal's retreat' is the definition.
(I've seen this before)
'atmosphere associated with large' is the wordplay.
'atmosphere' becomes 'air' (synonyms).
'associated with' is a charade indicator (letters next to each other).
'large' becomes 'L' (eg in clothes sizes).
'air' put after 'l' is 'LAIR'.
(Other definitions for lair that I've seen before include "Supposed large ape, the Bigfoot" , "Rail around hiding-place" , "Burrow" , "Wild animal's retreat" , "Place to hide" .)