Miserable face sergeant major wears (6)
I believe the answer is:
dismal
'miserable' is the definition.
(both can mean melancholy)
'face sergeant major wears' is the wordplay.
'face' becomes 'dial' (face of a clock).
'sergeant major' becomes 'SM' (abbreviation for sergeant major).
'wears' means one lot of letters goes inside another.
'dial' going around 'sm' is 'DISMAL'.
(Other definitions for dismal that I've seen before include "Gloomy and cheerless" , "Gloomy, dreary" , "dark" , "Lugubrious" , "Showing gloom, dreary" .)