Arboreal marsupial (5)
(Other definitions for phalanger that I've seen before include "may be flying in Australia" , "Lemur-like creature" , "Little Aussie's" , "Marsupial" .)
'arboreal marsupial' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for koala that I've seen before include "Arboreal Australian native" , "Marsupial "bear"" , "Marsupial climber" , "Australian tree-climbing marsupial with grey fur and big ears" , "Aussie tree-climber" .)
(Other definitions for possum that I've seen before include "Thick-furred marsupial" , "Small furry Australian marsupial" , "Tree-dwelling Australasian marsupial" , "The Australasian phalanger" , "Play -- (pretend to be dead)" .)