Island originally home to climbing marsupial — one of these is as dead as a dodo? (9)

'one of these is as dead as a dodo?' is the definition.
(as in the phrase 'dead as a doornail')

'island originally home to climbing marsupial' is the wordplay.
'originally' indicates anagramming the letters.
'home to' indicates putting letters inside (some letters house or home others).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'marsupial' becomes 'roo' (short for kangaroo).
'roo' written backwards gives 'oor'.
'island' is an anagram of 'dnails'.
'dnails' going around 'oor' is 'DOORNAILS'.