Concealed overdue books (6)
I believe the answer is:
latent
'concealed' is the definition.
(thesaurus)
'overdue books' is the wordplay.
'overdue' becomes 'late' ('late' can be similar in meaning to 'overdue').
'books' becomes 'nt' (New Testament).
'late'+'nt'='LATENT'
(Other definitions for latent that I've seen before include "Hidden, dormant" , "Existing but not yet active, say talent" , "waiting to be discovered" , "Unrealised, potential" , "Existing but hidden" .)