Most gloomy shoot -- describing Ken Loach film (7)
I believe the answer is:
darkest
'most gloomy' is the definition.
('dark' can be similar in meaning to 'gloomy')
'shoot describing ken loach film' is the wordplay.
'shoot' becomes 'dart' ('dart' can be a synonym of 'shoot').
'describing' means one lot of letters goes inside another (describe can mean to form the outline of).
'ken loach film' becomes 'kes'.
'dart' going around 'kes' is 'DARKEST'.
(Other definitions for darkest that I've seen before include "Gloomiest or blackest" , "'The . . . . . . . hour comes before the dawn'" , "Most devoid of light" , "Most gloomy" , "Most dim" .)