One has to tax a coral structure (5)
I believe the answer is:
atoll
'a coral structure' is the definition.
(I've seen this in another clue)
'one has to tax' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'has' is a charade indicator (letters next to each other).
'to tax' becomes 'toll' (I've seen this in other clues).
'a'+'toll'='ATOLL'
(Other definitions for atoll that I've seen before include "Coral island with central lagoon" , "String of islands" , "Key" , "Bikini for one" , "land threatened by rising sea levels" .)