Engine external to the boat (8)
I believe the answer is:
outboard
'engine external to the boat' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for outboard that I've seen before include "Engine mounted at the stern of a small boat" , "Small boat's motor" , "Type of boat engine" , "Sort of motor" , "Engine external to the boat" .)