Musical works about New Testament — a task unfinished (8)
I believe the answer is:
cantatas
'musical works' is the definition.
(cantata is a type of musical composition)
'about new testament a task unfinished' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'new testament' becomes 'nt'.
'unfinished' means to remove the last letter.
'task' with its last letter taken away is 'tas'.
'ca'+'nt'+'a'+'tas'='CANTATAS'
(Other definitions for cantatas that I've seen before include "Musical works sung by chorus and soloists" , "Vocal pieces" .)