Commuters failing to get Metro for film (4)

I believe the answer is:

scum

I'm a little stuck... Click here to teach me more about this clue!

'for film' is the definition.
(scum is a kind of film)

I don't understand the rest of the clue.

(Other definitions for scum that I've seen before include "Algae" , "to be seen on top" , "low life" , "Dirty surface of a liquid" , "Floating dirt on surface" .)

I've seen this clue in The Irish Times.

Want a hint initially instead of a full solution? Install my app