Island's fortune reversed by a lucky leader (5)
I believe the answer is:
atoll
'island's' is the definition.
(thesaurus)
'fortune reversed by a lucky leader' is the wordplay.
'fortune' becomes 'lot' ('lot' can be a synonym of 'fortune'**).
'reversed' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'leader' suggests taking the first letters.
The initial letter of 'lucky' is 'l'.
'lot' written backwards gives 'tol'.
'tol' after 'a' is 'atol'.
'atol'+'l'='ATOLL'
(Other definitions for atoll that I've seen before include "Circular coral reef" , "Coral island with central lagoon" , "tropical island" , "Key" , "Ring-shaped coral reef enclosing a lagoon" .)