Say it's a cinch, in your hearing (6)
I believe the answer is:
assert
'say' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(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" .)