Paintings have captured a horse race on island (9)
I believe the answer is:
tahitians
'race on island' is the definition.
(the people of Tahiti)
'paintings have captured a horse' is the wordplay.
'paintings' becomes 'titians' (paintings by Titian).
'have captured' is an insertion indicator.
'horse' becomes 'h' (synonyms).
'a'+'h'='ah'
'titians' placed around 'ah' is 'TAHITIANS'.
(Other definitions for tahitians that I've seen before include "Islanders in the 1 ac" , "Polynesian types" , "some painted by Gauguin" , "some 16 [ISLANDER] s" , "race on island" .)