Certain contentment after dismissal of plea (4)

I believe the answer is:
sure
'certain' is the definition.
(synonyms)
I cannot 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 "Safe" , "Completely confident that one is right" , "Definite" , "Confident sort of ruse" , "Agreed" .)
