Ms Farrow's mother creating an unwholesome atmosphere? (6)
I believe the answer is:
miasma
'an unwholesome atmosphere?' is the definition.
(miasma can mean an unpleasant atmosphere)
'ms farrow's mother' is the wordplay.
'ms farrow's' becomes 'Mia's' (actress Mia Farrow).
'mother' becomes 'ma' (ma can mean mother).
'mias'+'ma'='MIASMA'
'creating' is the link.
(Other definitions for miasma that I've seen before include "Noxious atmosphere" , "Unpleasant smell/vapour" , "Foul vapours or unwholesome air" , "Bad air" , "Noxious vapour" .)