Start motorboat (6)
I believe the answer is:
launch
I believe this is a double definition.
'start' is the first definition.
(launching is a kind of starting)
'motorboat' is the second definition.
(I know that launch is a type of motorboat)
(Other definitions for launch that I've seen before include "Put a new product on the market" , "Set up - found" , "starter?" , "Send off (a rocket)" , "Set (a boat) afloat" .)