Atmosphere destroyed clematis mostly (7)
I believe the answer is:
climate
'atmosphere' is the definition.
(I've seen this before)
'destroyed clematis mostly' is the wordplay.
'destroyed' indicates anagramming the letters.
'mostly' means to remove the last letter (most but not all of the word).
'clematis' with its last letter taken away is 'clemati'.
'clemati' with letters rearranged gives 'CLIMATE'.
(Other definitions for climate that I've seen before include "Long-term prevalent weather conditions" , "Prevailing state" , "General weather conditions in an area" , "Atmosphere" , "Came lit (anag.)" .)