Cheese a fly will not be on for long! (4)

I believe the answer is:

brie

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'cheese' is the definition.
(I know that Brie is a cheese)

I don't understand how the rest of the clue works.

(Other definitions for brie that I've seen before include "Kind of soft cheese" , "dairy product" , "Soft, creamy French cheese" , "cheesy stuff" , "French produce" .)

I've seen this clue in the Evening Standard.

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