"That's a fact" (4)
I believe the answer is:
true
'that's a fact' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for true that I've seen before include "Honest - expression of agreement" , "Genuine, real" , "Honest and straight" , "Exact; faithful" , "Dependable -- loyal" .)