Twin-hulled vessel (9)
I believe the answer is:
catamaran
'twin-hulled vessel' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for catamaran that I've seen before include "Boat with twin, parallel hulls" , "Twin-hulled vessel" , "raft of logs" , "Sailing vessel" , "One vessel" .)