A charge for crossing island (5)
I believe the answer is:
atoll
'island' is the definition.
(I know that atoll is a type of island)
'a charge for crossing' is the wordplay.
'charge for crossing' becomes 'toll' (as in a bridge toll).
'a'+'toll'='ATOLL'
(Other definitions for atoll that I've seen before include "Ring of coral" , "tropical paradise?" , "Land around a lagoon, such as Bikini" , "Coral island(s) surrounding a lagoon" , "Bikini, perhaps" .)