Hostility in opossum in Australia when climbing (6)
I believe the answer is:
animus
'hostility' is the definition.
(animus is a kind of hostility)
'in opossum in australia when climbing' is the wordplay.
'in' indicates the answer is hidden within the clue.
'when climbing' is a reversal indicator.
'sumina' is hidden within 'opossum in australia'.
'sumina' backwards is 'ANIMUS'.
(Other definitions for animus that I've seen before include "Hostile spirit in speech or action" , "Bad blood" , "Hostility, ill-feeling" , "Feeling of strong dislike" , "US man I turn to with hostility" .)