Boat set in motion (6)
I believe the answer is:
launch
This clue is a double definition.
'boat' is the first definition.
(launch is a kind of boat)
'set in motion' is the second definition.
(synonyms)
(Other definitions for launch that I've seen before include "Power-driven boat" , "Inaugurate" , "Send off (a missile)" , "Initiate; a boat" , "'Send into water or into space, say (6)'" .)