Melancholy detective, without love (6)
I believe the answer is:
morose
'melancholy' is the definition.
(thesaurus)
'detective without love' is the wordplay.
'detective' becomes 'morse' (I've seen this before).
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'love' becomes 'o' (In tennis, 'love' means 'zero').
'morse' enclosing 'o' is 'MOROSE'.
(Other definitions for morose that I've seen before include "Miserable" , "dark" , "Sullen, crabby" , "Gloomy and unsociable" , "Down in the dumps" .)