Join up for a singular course (4)

I believe the answer is:

link

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'join' is the definition.
(I know that join can be written as link)

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

(Other definitions for link that I've seen before include "section of chain" , "Physically join" , "Item of mail" , "Tie-up" , "Connecting part" .)

I've seen this clue in the Evening Standard.

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