Unpleasant atmosphere when old lady goes behind this writer's back (6)
I believe the answer is:
miasma
'unpleasant atmosphere' is the definition.
(miasma is an unpleasant or noxious vapour)
'when old lady goes behind this writer's back' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'old lady' becomes 'ma' (both can mean mother).
'goes behind' says to put letters next to each other.
'this writer's' becomes 'I'm' ('I am' referring to the writer of the clue).
'back' says the letters should be written backwards.
'im' backwards is 'mi'.
'as'+'ma'='asma'
'asma' put after 'mi' is 'MIASMA'.
(Other definitions for miasma that I've seen before include "Noxious atmosphere" , "Unpleasant vapour or influence" , "Unwholesome atmosphere, unhealthy vapors" , "Effluvium" , "Pollution in the atmosphere" .)