Racing boat (5)
'racing boat' is the definition.
(scull is a kind of racing boat)
This is all the clue.
(Other definitions for scull that I've seen before include "Small oar" , "Row with two oars" , "Light rowing boat" , "Blade" , "racing shell" .)
(Other definitions for yacht that I've seen before include "Pleasure craft" , "Sailing or motor vessel for cruising or racing" , "Luxury purchase" , "Sailing boat" , "Kind of boat" .)
(Other definitions for sloop that I've seen before include "Sailboat" , "one going to sea" , "One-masted sailing ship" , "One-masted sailing boat" , "Sailing vessel with one mast" .)