Nasty-sounding appearance (4)
I believe the answer is:
mien
'appearance' is the definition.
(I've seen this in another clue)
'nasty-sounding' is the wordplay.
'nasty' becomes 'mean' (similar in meaning).
'sounding' shows a homophone (sound like).
'mean' is a homophone of 'MIEN'.
(Other definitions for mien that I've seen before include "aspect" , "Facial expression sounds miserly" , "Air - look - manner" , "A person's demeanour" , "Bearing, demeanour" .)