500 idle, maybe, for spell in Caribbean (6)
I believe the answer is:
voodoo
'spell in caribbean' is the definition.
(spell as in magic - voodoo is from the Caribbean)
I can't explain the remainder of the clue.
(Other definitions for voodoo that I've seen before include "Caribbean magic cult" , "Form of magic" , "Jinx" , "Black magid from West Indies" , "Religious cult involving witchcraft" .)