Most of the small island rebuffed vessel (5)
I believe the answer is:
yacht
'vessel' is the definition.
(I know that yacht is a type of vessel)
'most of the small island rebuffed' is the wordplay.
'most of' means to remove the last letter.
'small island' becomes 'cay' (I've seen this in another clue).
'rebuffed' says the letters should be written in reverse.
'the' with its final letter taken off is 'th'.
'th'+'cay'='thcay'
'thcay' backwards is 'YACHT'.
(Other definitions for yacht that I've seen before include "Type of boat" , "Light, fast sailing-vessel" , "Pleasure-boat" , "Sailboat" , "Sailing-boat" .)