Dull affair having no end (5)

I believe the answer is:

matte

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'dull' is the definition.
(similar in meaning)

I can't explain the remainder of the clue.

(Other definitions for matte that I've seen before include "Mask used to block out areas of the image in filming" , "Dull, flat" , "Unusually dull" , "A dull finish" , "Mask used in film; ET mat (anag.)" .)

I've seen this clue in The Guardian and The Independent.

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