Really popular introduction to this bible book (2,5)
I believe the answer is:
in truth
'really' is the definition.
(I know that really can be written as in truth)
'popular introduction to this bible book' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'introduction to' says to take the initial letters.
'bible book' becomes 'ruth' (I've seen this in another clue).
The initial letter of 'this' is 't'.
'in'+'t'+'ruth'='IN TRUTH'
(Other definitions for in truth that I've seen before include "Honestly" , "As a matter of fact" , "To be honest" , "Really" , "Actually" .)