A tax on some reef (5)
I believe the answer is:
atoll
'some reef' is the definition.
(atoll is a kind of reef)
'a tax on' is the wordplay.
'tax' becomes 'toll' (I've seen this in other clues).
'on' means one lot of letters go next to another.
'a'+'toll'='ATOLL'
(Other definitions for atoll that I've seen before include "Bikini, say" , "Coral isle" , "Ring-shaped coral reef enclosing a lagoon" , "Ring-shaped coral island" , "String of islands" .)