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