To be so is agreeable (4)

I believe the answer is:

sure

I'm a little stuck... Click here to teach me more about this clue!

But I can't explain it at all! Could you help me understand?

(Other definitions for sure that I've seen before include "Completely confident that one is right" , "Completely certain" , "See 12" , "stable" , "Definite" .)

Want a hint initially instead of a full solution? Install my app