Vessel to put to sea (6)
I believe the answer is:
launch
'vessel' is the definition.
(launch is a type of motorboat)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for launch that I've seen before include "Send on course, ship or book" , "Establish" , "Send on its course, new ship or publication" , "Begin; motor boat" , "Send a vessel into water" .)