Single-masted vessel
I believe the answer is:
'single-masted vessel' 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 "Vessel with a single mast" , "One-masted sailing boat" , "'----- John B', 1966 Beach Boys' hit" , "something that's rigged" , "One-masted sailing ship" .)