Extra item a divine conclusion? I’m not sure (8)
I believe the answer is:
addendum
'extra item' is the definition.
(I've seen this before)
'a divine conclusion? i'm not sure' is the wordplay.
'divine' becomes 'dd' (divine can mean theologian eg Doctor of Divinity).
'conclusion?' becomes 'end' (end is a kind of conclusion).
'i'm not sure' becomes 'um' (expression of hesitation).
'a'+'dd'+'end'+'um'='ADDENDUM'
(Other definitions for addendum that I've seen before include "Dud ad-men dealt with in end of book" , "Addition at end of book" , "Thing to be added" , "Item appended to book" , "Supplementary text tacked on to the end" .)