Gloomy and slow-witted (3)
I believe the answer is:
dim
This clue is a double definition.
'gloomy' is the first definition.
(thesaurus)
'slow-witted' is the second definition.
(I've seen this in another clue)
'and' acts as a link.
(Other definitions for dim that I've seen before include "Somewhat out of sight" , "Not very bright or intelligent" , "Not bright or distinct" , "progressing towards piano" , "Unpromising" .)