Say it’s a cinch, for all to hear (6)
I believe the answer is:
assert
'say' is the definition.
(I know that say is a more specific form of the action assert)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for assert that I've seen before include "State firmly and with conviction" , "State firmly, aver" , "Maintain - allege" , "Affirm, allege" , "Declare, state vigorously" .)