Three-hulled vessel (8)
I believe the answer is:
trimaran
'three-hulled vessel' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for trimaran that I've seen before include "Three-hulled vessel" , "Ocean vessel with three hulls side by side" , "Three-hulled boat" , "Three-hulled yacht" , "Sailing vessel" .)