City noise behind sand hill (7)
I believe the answer is:
dunedin
'city' is the definition.
(I've seen this before)
'noise behind sand hill' is the wordplay.
'noise' becomes 'din' (din can mean a loud noise or disturbance).
'behind' means one lot of letters go next to another.
'sand hill' becomes 'dune' (a dune is a hill of sand).
'din' put after 'dune' is 'DUNEDIN'.
(Other definitions for dunedin that I've seen before include "New Zealand location" , "New Zealand port" , "Antipodean seaport" , "NZ South Island port" , "Otago port" .)