Powerless South American island quietly coming up (9)
I believe the answer is:
incapable
'powerless' is the definition.
(thesaurus)
'south american island quietly coming up' is the wordplay.
'south american' becomes 'inca' (the Inca were a South American civilization).
'island' becomes 'elba' (Italian island).
'quietly' becomes 'p' (p for piano and quiet).
'coming up' shows that the letters should be reversed in order (in a down clue, letters go up).
'elba'+'p'='elbap'
'elbap' reversed gives 'pable'.
'inca'+'pable'='INCAPABLE'
(Other definitions for incapable that I've seen before include "hopeless" , "Unable, incompetent" , "lacking ability" , "Useless" , "See possibly drunken" .)