Gloomy doctor on organ (5)
I believe the answer is:
drear
'gloomy' is the definition.
(I know that gloomy can be written as drear)
'doctor on organ' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'organ' becomes 'ear' (the ear is an organ).
'dr'+'ear'='DREAR'
(Other definitions for drear that I've seen before include "Dismal, dull" , "Poetic dull and uninteresting" , "miserable" , "Dull and tedious" , "Dismal (literary)" .)