Tax one leaving coral island (4)
I believe the answer is:
toll
'tax' is the definition.
(I've seen this in other clues)
'one leaving coral island' is the wordplay.
'one' becomes 'a'.
'leaving' is a deletion indicator.
'coral island' becomes 'atoll' (I've seen this before).
'atoll' with 'a' taken out is 'TOLL'.
(Other definitions for toll that I've seen before include "Charge (to use road)" , "Be inclined" , "Sum payable to use main road (4)" , "number of casualties" , "Bridge, tunnel fee" .)