Atmosphere around sort of valley (3)
I believe the answer is:
ria
'sort of valley' is the definition.
'atmosphere around' is the wordplay.
'atmosphere' becomes 'air' ('air' can be a synonym of 'atmosphere').
'around' says the letters should be written in reverse.
'air' backwards is 'RIA'.
(Other definitions for ria that I've seen before include "Institution for the promotion of art, in short" , "Little girl" , "Submerged valley" , "Coastal inlet" , "Irish academy for sciences and humanities, in short" .)