Reality; certainty (4)
I believe the answer is:
fact
I believe this clue is a double definition.
'reality' is the first definition.
(I know that fact is a type of reality)
'certainty' is the second definition.
(I've seen this in another clue)
(Other definitions for fact that I've seen before include "Not fake news!" , "An indisputable truth" , "copyright protection agency?" , "given" , "True statement" .)