A brief denial (4)

I believe the answer is:

long

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'brief denial' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?

I cannot understand how the remainder of the clue works.

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(Other definitions for long that I've seen before include "hunger" , "Pant" , "bought in advance" , "to be tall" , "summer break?" .)

I've seen this clue in the Evening Standard.

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