Fact (5)
'fact' is the definition.
(singular form of 'data')
This is all the clue.
(Other definitions for datum that I've seen before include "Basic fact" , "Unit of dope" , "Fixed starting point for measurements" , "Single informative fact" , "Fact or stat" .)
'fact' is the definition.
(I know that truth is a type of fact)
This is the entire clue.
(Other definitions for truth that I've seen before include "Veracity; established fact" , "known facts" , "Gospel" , "Candour" , "real story" .)
'fact' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for reality that I've seen before include "The way things actually are" , "Verisimilitude" , "Actual existence" , "Truth, actuality" , "The way things are, no illusions" .)