Showing a dirty film (5)

I believe the answer is:

dusty

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'dirty film' is the definition.
'dusty' can be an answer for 'dirty' (I've seen this before). I am not certain of the 'film' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for dusty that I've seen before include "Grubby" , "In need of cleaning" , "Boring" , "Study (anag.)" , "In need of a spring clean" .)

I've seen this clue in the Evening Standard.

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