Complete set of books kept in European republic (6)
I believe the answer is:
entire
'complete' is the definition.
(synonyms)
'set of books kept in european republic' is the wordplay.
'set of books' becomes 'nt' (New Testament).
'kept in' is an insertion indicator.
'european republic' becomes 'Eire' (Irish name for Ireland).
'nt' put within 'eire' is 'ENTIRE'.
(Other definitions for entire that I've seen before include "Envelope" , "everything" , "outright" , "excluding nothing" , "Unmitigated" .)