Settler on island breaking horse (8)
I believe the answer is:
colonist
'settler' is the definition.
('colonist' can be a synonym of 'settler')
'on island breaking horse' is the wordplay.
'island' becomes 'is'.
'breaking' is an insertion indicator.
'horse' becomes 'colt' (I've seen this before**).
'on'+'is'='onis'
'onis' placed within 'colt' is 'COLONIST'.
(Other definitions for colonist that I've seen before include "I'll settle" , "Immigrant" , "Settler" , "Coin slot (anag.)" .)