Say it’s a sure thing (6)

I believe the answer is:

assert

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'say' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for assert that I've seen before include "State firmly or vigorously" , "Declare or affirm" , "Stares (anag)" , "Insist upon (rights etc)" , "Forcefully state" .)

I've seen this clue in the Evening Standard.

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