Head of possum, strange arboreal animal (5,4)
I believe the answer is:
polar bear
'animal' is the definition.
(thesaurus)
'head of possum strange arboreal' is the wordplay.
'head of' says to take the initial letters.
'strange' is an anagram indicator.
The initial letter of 'possum' is 'p'.
'arboreal' anagrammed gives 'olarbear'.
'p'+'olarbear'='POLAR BEAR'
(Other definitions for polar bear that I've seen before include "Arctic mammal" , "Great white animal of the Arctic" , "Arctic creature" , "Largest carnivorous land mammal" , "Arctic pack ice resident" .)