Insist a sure thing is to be heard (6)
I believe the answer is:
assert
'insist' is the definition.
(asserting is a kind of insisting)
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 "Declare, claim" , "State firmly, aver" , "Declare or affirm" , "State firmly or vigorously" , "Declare, state vigorously" .)