Gloomy sergeant major in control (6)
I believe the answer is:
dismal
'gloomy' is the definition.
(I know that gloomy can be written as dismal)
'sergeant major in control' is the wordplay.
'sergeant major' becomes 'SM' (abbreviation for sergeant major).
'in' means one lot of letters goes inside another.
'control' becomes 'dial' (dialing is a kind of controlling).
'sm' put into 'dial' is 'DISMAL'.
(Other definitions for dismal that I've seen before include "Sad, gloomy" , "down in the dumps" , "Depressing; dreary" , "Feeble" , "Dreary; awful" .)