Certain, sure (8)
I believe the answer is:
definite
This clue is a double definition.
'certain' is the first definition.
(similar in meaning)
'sure' is the second definition.
(synonyms)
(Other definitions for definite that I've seen before include "The is the article" , "Certain; clearly stated" , "Positive, certain" , "Clearly decided" , "Known for certain" .)