A little boat often leaves a coral island (5)
I believe the answer is:
atoll
'coral island' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for atoll that I've seen before include "Bikini, perhaps" , "land threatened by rising sea levels" , "Coral island with central lagoon" , "Coral isle" , "Land around a lagoon, such as Bikini" .)