The most common and most likely answer to this crossword clue is the 8 plus 3 plus 1 letter word WITHNAIL AND I
W
I
T
H
N
A
I
L
A
N
D
I
Buddy explains!
The straight part
We know that "Dark film" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dark film = withnail and i.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.