A New Testament worker
I believe the answer is:
ant
'worker' is the definition.
(worker is a type of ant)
'a new testament' is the wordplay.
'new testament' becomes 'nt'.
'a'+'nt'='ANT'
(Other definitions for ant that I've seen before include "A soldier, for one" , "Colonial insect?" , "Member of colony" , "One working hard" , "Small social insect" .)