Holiday island’s brought in neat organisation of groups (9)
I believe the answer is:
tribalism
'organisation of groups' is the definition.
(organisation or arrangement into tribes)
'holiday island's brought in neat' is the wordplay.
'holiday island's' becomes 'balis' (Bali is a popular holiday destination).
'brought in' is an insertion indicator.
'neat' becomes 'trim' (synonyms).
'balis' inserted within 'trim' is 'TRIBALISM'.
(Other definitions for tribalism that I've seen before include "ethnic customs" , "allegiance to group" , "divisive tendency" , "Organisation of society on ethnic lines" , "cultural identity?" .)