Dark film genre (4)
I believe the answer is:
'dark film genre' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for noir that I've seen before include "(Of film or fiction) with bleak settings and cynical characters" , "Kind of dark film" , "cinematic style" , "Dark in France, like brooding films of 40s and 50s" , "type of film" .)