Exotic island monkey astride a horse (6)
I believe the answer is:
tahiti
'exotic island' is the definition.
(I've seen this before)
'monkey astride a horse' is the wordplay.
'monkey' becomes 'titi' (titi is a kind of monkey).
'astride' is an insertion indicator.
'horse' becomes 'h' (synonyms).
'a'+'h'='ah'
'titi' placed around 'ah' is 'TAHITI'.
(Other definitions for tahiti that I've seen before include "Main island of French Polynesia" , "I hit at this exotic island" , "French Polynesian island" , "I hit at the tropical island" , "Romantic Polynesian island, capital Papeete" .)