Marsupial group on island with aquatic bird (9)
I believe the answer is:
bandicoot
'marsupial' is the definition.
(bandicoot is a kind of marsupial)
'group on island with aquatic bird' is the wordplay.
'group' becomes 'band' (band can mean a group of people).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'island' becomes 'i' (geographical abbreviation).
'with' means one lot of letters go next to another.
'aquatic bird' becomes 'coot' (coot is a kind of aquatic bird).
'band'+'i'+'coot'='BANDICOOT'
(Other definitions for bandicoot that I've seen before include "Aussie native" , "Little animal" , "Insect-eating marsupial of Australia and New Guinea" , "Australian marsupial with long pointed muzzle and tail" , "Australian native" .)