Cold atmosphere over new landmark (5)
I believe the answer is:
cairn
'landmark' is the definition.
(a pile of stones)
'cold atmosphere over new' is the wordplay.
'cold' becomes 'c' (eg on taps).
'atmosphere' becomes 'air' (synonyms).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'c'+'air'+'n'='CAIRN'
(Other definitions for cairn that I've seen before include "Agree" , "Stones piled up as a marker" , "Mound of stones on mountain top" , "Heap of stones - terrier" , "Terrier - mound of stones" .)