Mixed metaphors back in the air (10)
I believe the answer is:
atmosphere
'air' is the definition.
('atmosphere' can be a synonym of 'air')
'mixed metaphors back in the' is the wordplay.
'mixed' is an anagram indicator.
'back in' suggests the final letters (the 'back' of the word).
The final letter of 'the' is 'e'.
'metaphors' with letters rearranged gives 'atmospher'.
'atmospher'+'e'='ATMOSPHERE'
(Other definitions for atmosphere that I've seen before include "We are always enveloped in this" , "prevailing mood" , "Pervading tone or mood" , "Gas surrounding a planet" , "character" .)