Say it's a cinch, in your hearing (6)

I believe the answer is:

assert

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

I can't explain the rest of the clue.

(Other definitions for assert that I've seen before include "State firmly and positively" , "Maintain formally" , "State firmly or vigorously" , "Declare or affirm" , "Declare, state vigorously" .)

I've seen this clue in the Evening Standard.

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