Beastly Aussie resembling Teddy (5)
I believe the answer is:
koala
'teddy' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'beastly aussie resembling' is the wordplay.
'beastly aussie' becomes 'ko' (I can't explain this - if you can you should believe this answer much more).
'resembling' becomes 'ala' (I've seen this in another clue).
'ko'+'ala'='KOALA'
Can you help me to learn more?
(Other definitions for koala that I've seen before include "Bear-like Australian marsupial" , "Arboreal Australian native" , "Aussie tree-climber" , "Australian creature" , "Arboreal marsupial" .)