Positive agreement (4)
I believe the answer is:
sure
'positive' is the definition.
(similar in meaning)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sure that I've seen before include "Certain to be a ruse" , "The old sewer" , "Completely certain" , "certainly in the USA" , "Safe" .)