Gloomy old detective's around (6)
I believe the answer is:
morose
'gloomy' is the definition.
(thesaurus)
'old detective's around' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'detective' becomes 'morse' (I've seen this before).
'around' indicates putting letters inside.
'o' placed inside 'morse' is 'MOROSE'.
(Other definitions for morose that I've seen before include "Gloomy and unsociable" , "Sullen and ill-tempered" , "dark" , "Sullen and moody" , "Sullen or glum." .)