Gloomy doctor holds one up (3)
I believe the answer is:
dim
'gloomy' is the definition.
(thesaurus)
'doctor holds one up' is the wordplay.
'doctor' becomes 'md' ('MD' can be a synonym of 'doctor'**).
'holds' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'md' going around 'i' is 'mid'.
'mid' in reverse letter order is 'DIM'.
(Other definitions for dim that I've seen before include "Not bright, shadowy" , "Unpromising" , "Unintelligent; badly lit" , "Faint (light)" , "Shady" .)