One tied at speed? (4)

I believe the answer is:

knot

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'speed?' is the definition.
(knot is a nautical measure of speed)

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

Can you help me to learn more?

(Other definitions for knot that I've seen before include "Assemblage" , "Tie, hitch" , "binder?" , "Small cluster - huddled group - sheepshank, for example" , "Measure of speed at sea" .)

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