Most important having rook in such dark films? (4)
I believe the answer is:
noir
'such dark films?' is the definition.
(as in films noir)
'most important having rook' is the wordplay.
'most important' becomes 'no i' (resembles No 1).
'having' says to put letters next to each other.
'rook' becomes 'R' (chess abbreviation).
'noi'+'r'='NOIR'
'in' is the link.
(Other definitions for noir that I've seen before include "Kind of dark film" , "(Of film or fiction) with bleak settings and cynical characters" , "Film featuring bleak settings and cynical characters" , "cinematic style" , "film genre" .)