Gloomy detective eating nothing (6)
I believe the answer is:
morose
'gloomy' is the definition.
(I've seen this in another clue)
'detective eating nothing' is the wordplay.
'detective' becomes 'Morse' (Inspector Morse, fictional detective).
'eating' means one lot of letters goes inside another.
'nothing' becomes 'O' (looks like zero - 0).
'morse' placed around 'o' is 'MOROSE'.
(Other definitions for morose that I've seen before include "Moody, glum" , "Rarely" , "Melancholy" , "Lugubrious" , "blue" .)