Single-master (5)
I believe the answer is:
sloop
'single-master' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sloop that I've seen before include "Sailing vessel with one mast" , "Vessel with a single mast" , "Single-masted sailing vessel" , "Type of sailing boat" , "something that's rigged" .)