It's true! (1,3,3,3)
(Another definition for i kid you not that I've seen is " Honest".)
'it's true' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for fact that I've seen before include "Truth, reality" , "True statement" , "It's certainly not fiction" , "Indisputable thing" , "Certain thing" .)