Joke found in New Testament work (5)
I believe the answer is:
nonet
'work' is the definition.
(musical work for nine musicians)
'joke found in new testament' is the wordplay.
'joke' becomes 'one' (as in 'that's a good one!').
'found in' means one lot of letters goes inside another.
'new testament' becomes 'nt'.
'one' inserted within 'nt' is 'NONET'.
(Other definitions for nonet that I've seen before include "Musical composition -- tonne (anag)" , "Band of nine?" , "Instrumental group of nine players" , "Piece of music for nine" , "Tonne (anag.); music for a group" .)