Gloomy detectives in burial ground (9)
I believe the answer is:
graveyard
'burial ground' is the definition.
(synonyms)
'gloomy detectives' is the wordplay.
'gloomy' becomes 'grave' (both can mean sombre).
'detectives' becomes 'Yard' (Scotland Yard, Metropolitan Police HQ).
'grave'+'yard'='GRAVEYARD'
'in' is the link.
(Other definitions for graveyard that I've seen before include "Final destination" , "Burial ground" , "Cemetery" , "'God's acre'" , "remains here" .)