Unusually deviant animal's a kind of marsupial (9,5)
I believe the answer is:
tasmanian devil
'a kind of marsupial' is the definition.
'unusually deviant animal's' is the wordplay.
'unusually' indicates anagramming the letters.
'deviant'+'animals'='deviantanimals'
'deviantanimals' with letters rearranged gives 'TASMANIAN DEVIL'.
(Other definitions for tasmanian devil that I've seen before include "This beast is certainly wild" , "Ferocious Australian marsupial" , "Vicious little Aussie" , "Ferocious animal" , "Creature" .)