Single-masted boat (5)
I believe the answer is:
sloop
'single-masted boat' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sloop that I've seen before include "Sailing vessel with one mast" , "one going to sea" , "One-masted sailing boat" , "'----- John B', 1966 Beach Boys' hit" , "Single-masted sailing boat" .)