I’m back since Mum is in unwholesome air (6)
I believe the answer is:
miasma
'unwholesome air' is the definition.
(I've seen this before)
'i'm back since mum' is the wordplay.
'back' shows that the letters should be reversed in order.
'since' becomes 'as' (both can mean 'because').
'mum' becomes 'ma' (both can mean mother).
'im' back-to-front is 'mi'.
'mi'+'as'+'ma'='MIASMA'
'is in' acts as a link.
(Other definitions for miasma that I've seen before include "An unpleasant or unhealthy smell or vapour" , "foul atmosphere" , "Unwholesome atmosphere or unhealthy vapours" , "Foul vapour" , "Unpleasant vapour or influence" .)