Gloomy detective imprisons leader of Opposition (6)
I believe the answer is:
morose
'gloomy' is the definition.
(thesaurus)
'detective imprisons leader of opposition' is the wordplay.
'detective' becomes 'morse' (I've seen this before).
'imprisons' indicates putting letters inside.
'leader of' indicates taking the first letters.
The initial letter of 'opposition' is 'o'.
'morse' placed around 'o' is 'MOROSE'.
(Other definitions for morose that I've seen before include "Peevish and gloomy" , "Lugubrious" , "Sullen. bad-tempered" , "grouchy" , "Gloomy and unsociable" .)