Say for sure (4)
I believe the answer is:
aver
'say for sure' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aver that I've seen before include "To declare or affirm solemnly as true" , "State firmly - even rave about" , "Say firmly, rave if necessary" , "being assertive" , "Maintain - state" .)