A New Testament insect (3)
I believe the answer is:
ant
'insect' is the definition.
(ant is a kind of insect)
'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" , "Proverbially hard-working insect" , "Emmet or pismire" , "one living in colonies" , "Small busy insect" .)