Immoral to ignore series of religious books some years (6)
I believe the answer is:
decade
'some years' is the definition.
(I've seen this before)
'immoral to ignore series of religious books' is the wordplay.
'immoral' becomes 'decadent' (I've seen this before).
'to ignore' suggests deleting specific letters.
'series of religious books' becomes 'NT' (New Testament).
'decadent' with 'nt' taken out is 'DECADE'.
(Other definitions for decade that I've seen before include "Group of ten things" , "Ten-year period" , "Time period" , "The noughties maybe" , "The 1960s, for example" .)