Overdue books hidden (6)
I believe the answer is:
latent
'hidden' is the definition.
(latent can mean hidden or unused)
'overdue books' is the wordplay.
'overdue' becomes 'late' (similar in meaning).
'books' becomes 'NT' (New Testament).
'late'+'nt'='LATENT'
(Other definitions for latent that I've seen before include "Hidden (potential)" , "Existing but not yet active, say talent" , "Hidden (talent)" , "Hidden, dormant" , "Not (yet) apparent" .)