A New Testament insect (3)
'insect' is the definition.
(ant is a kind of insect)
'a new testament' is the wordplay.
'new testament' becomes 'nt'.
(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".)