A certainty of course (4,5)
I believe the answer is:
sure thing
I believe this is a double definition.
'a certainty' is the first definition.
(I've seen this before)
'of course' is the second definition.
(I've seen this before)
(Other definitions for sure thing that I've seen before include "no chance of losing" , "A certainty - I agree!" , "cert" , "of course" .)