Island where three houses sound upside down (8)
I believe the answer is:
trinidad
'island' is the definition.
(Trinidad is an example)
'three houses sound upside down' is the wordplay.
'three' becomes 'triad' (synonyms).
'houses' means one lot of letters goes inside another.
'sound' becomes 'din' (din can mean a loud disturbance).
'upside down' is a reversal indicator.
'din' backwards is 'nid'.
'triad' going around 'nid' is 'TRINIDAD'.
'where' acts as a link.
(Other definitions for trinidad that I've seen before include "West Indian island paired with Tobago" , "Caribbean island" , "W. I. island" , "Island linked to Tobago" , "English-speaking island off Venezuela" .)