One old book? After set of newer ones, frankly (2,5)
I believe the answer is:
in truth
'frankly' is the definition.
(synonyms)
'one old book? after set of newer ones' is the wordplay.
'one old book?' becomes 'ruth' (book of the Old Testament).
'after' says to put letters next to each other.
'set of newer ones' becomes 'int' (I can't explain this - if you can you should believe this answer much more).
'ruth' after 'int' is 'IN TRUTH'.
(Other definitions for in truth that I've seen before include "Honestly" , "To be honest" , "As a matter of fact" , "Really" , "Actually" .)