Truth; reality (4)
I believe the answer is:
fact
I believe this clue is a double definition.
'truth' is the first definition.
(fact can mean truth or reality)
'reality' is the second definition.
(fact is a kind of reality)
(Other definitions for fact that I've seen before include "given" , "Statement of verifiable information" , "It's certainly not fiction" , "Not fake news!" , "Item of truth" .)