Saturnine old detective outside (6)
I believe the answer is:
morose
'saturnine' is the definition.
('morose' can be a synonym of 'saturnine')
'old detective outside' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'detective' becomes 'morse' (I've seen this before).
'outside' is an insertion indicator.
'o' inserted within 'morse' is 'MOROSE'.
(Other definitions for morose that I've seen before include "Down in the dumps" , "in gloomy mood" , "Sullen and gloomy" , "Sullen, crabby" , "Moody, glum" .)