Racing boats (6)
(Other definitions for yachts that I've seen before include "Pleasure craft" , "Pleasue boats" , "Pleasure crafts" , "Sailing boats" , "Racing or cruising boats" .)
'racing boats' is the definition.
(scull is a kind of racing boat)
This is all the clue.
(Other definitions for sculls that I've seen before include "Pair of small oars" , "Pair of short oars used by single oarsman" , "Oars; race" , "Small oars used by single rower" , "Small oars used by single rowers" .)