Start of demolition in built-up area that is on the Indian Ocean (6)
I believe the answer is:
durban
'that is on the indian ocean' is the definition.
(city on the Indian Ocean)
'start of demolition in built-up area' is the wordplay.
'start of' indicates taking the first letters.
'in built-up area' becomes 'urban'.
The initial letter of 'demolition' is 'd'.
'd'+'urban'='DURBAN'
(Other definitions for durban that I've seen before include "Largest city in KwaZulu-Natal" , "in Natal" , "Rub Dan up the wrong way in S.Africa" , "Southern port" , "South African city port" .)