Space to fasten a boat up (4)

I believe the answer is:

room

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'space' is the definition.
(enough space or room to do something)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for room that I've seen before include "Enclosed part of building" , "Space within a building" , "Latitude" , "Place" , "Box ---, storage area" .)

I've seen this clue in The Globe and Mail.

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