New Testament and Old Testament book written by first person, really (2,5)
I believe the answer is:
in truth
'really' is the definition.
(I know that really can be written as in truth)
'new testament and old testament book written by first person' is the wordplay.
'new testament' becomes 'nt'.
'and' means one lot of letters go next to another.
'old testament book' becomes 'Ruth' (Ruth is a book of the Old Testament).
'written by' says to put letters next to each other.
'first person' becomes 'I' (first-person pronoun).
'nt' put next to 'ruth' is 'ntruth'.
'ntruth' after 'i' is 'IN TRUTH'.
(Other definitions for in truth that I've seen before include "Honestly" , "As a matter of fact" , "To be honest" , "Actually" , "Really" .)