Boat about to leave for Caribbean island (6)
I believe the answer is:
tobago
'caribbean island' is the definition.
(I've seen this before)
'boat about to leave' is the wordplay.
'about' indicates an anagram.
'to' means one lot of letters go next to another.
'leave' becomes 'go' (**).
'boat' is an anagram of 'toba'.
'toba'+'go'='TOBAGO'
'for' acts as a link.
(Other definitions for tobago that I've seen before include "Trinidad companion island" , "Island paired with Trinidad" , "Trinidad's smaller partner island" , "West Indian island" , "Island in West Indies" .)