An Australian digger (6)
I believe the answer is:
wombat
'an australian digger' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for wombat that I've seen before include "Australian animal like small bear" , "Burrowing animal" , "Australian animal resembling small bear" , "Marsupial of Oz" , "Australian burrowing marsupial" .)