Vessel that's off-course? (7)

I believe the answer is:

drifter

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'vessel' is the definition.
(I know that drifter is a type of fishing vessel)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for drifter that I've seen before include "Itinerant type" , "Fishing-boat" , "Wanderer; fishing vessel" , "Vagrant - fishing boat" , "Fishing boat; nomad" .)

I've seen this clue in The Telegraph.

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