Cold atmosphere at new stone mound (5)
I believe the answer is:
cairn
'stone mound' is the definition.
(I've seen this before)
'cold atmosphere at new' is the wordplay.
'cold' becomes 'c' (eg on taps).
'atmosphere' becomes 'air' ('air' can be a synonym of 'atmosphere'**).
'at' means one lot of letters go next to another.
'new' becomes 'n'.
'c'+'air'+'n'='CAIRN'
(Other definitions for cairn that I've seen before include "Stone-pile landmark" , "Small terrier" , "Heap of stones on mountain-top" , "Heap of stones - terrier" , "Stone mound" .)