It's a fact (5)
'it's a fact' is the definition.
(singular form of 'data')
This is all the clue.
(Other definitions for datum that I've seen before include "One item of information" , "given to the Romans" , "Given point or position" , "Single informative fact" , "Single piece of information" .)
'it's a fact' is the definition.
(truth is a kind of fact)
This is the entire clue.
(Other definitions for truth that I've seen before include "Gospel" , "Candour" , "Verity" , "Belief accepted as correct" , "Beauty is (said Keats)" .)