Vessel set in motion (6)
I believe the answer is:
launch
I believe this clue is a double definition.
'vessel' is the first definition.
(launch is a type of motorboat)
'set in motion' is the second definition.
(launching is a kind of setting in motion)
(Other definitions for launch that I've seen before include "'Send into water or into space, say (6)'" , "Send on course, ship or book" , "starter?" , "Send off (a rocket)" , "(Send) motor-boat (into the water)" .)