Extremely gripping - it's a fact (6)
I believe the answer is:
verity
'a fact' is the definition.
(verity is a kind of fact)
'extremely gripping it's' is the wordplay.
'extremely' becomes 'very' (I've seen this before).
'gripping' means one lot of letters goes inside another.
'it's' becomes 'it' (I've seen this in other clues).
'very' enclosing 'it' is 'VERITY'.
(Other definitions for verity that I've seen before include "Truthfulness" , "Truth - girl's name" , "A lady" .)