Quality of hearing reduced (4)
I believe the answer is:
aura
'quality' is the definition.
(aura is a kind of quality)
'of hearing reduced' is the wordplay.
'of hearing' becomes 'aural' (aural means relating to hearing).
'reduced' means to remove the last letter.
'aural' with its last letter taken away is 'AURA'.
(Other definitions for aura that I've seen before include "Distinctive character, essence of a living thing" , "Atmosphere [ital]" , "Atmosphere, quality" , "Occult glow" , "Perception of distinctive personality" .)