African expresses disapproval on island (5)
I believe the answer is:
tutsi
'african' is the definition.
(the Tutsi are an African ethnic group)
'expresses disapproval on island' is the wordplay.
'expresses disapproval' becomes 'tuts' (tutting is a disapproving sound).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'island' becomes 'i' (geographical abbreviation).
'tuts'+'i'='TUTSI'
(Other definitions for tutsi that I've seen before include "Tribespeople" , "African tribe" , "Member of a Bantu people of Rwanda and Burundi" , "Rwandan group" , "Bantu speaking people of Rwanda and Burundi" .)