Join up, pain though it is (6)

I believe the answer is:

stitch

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'join' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for stitch that I've seen before include "Pain in the side brought on by exercise" , "Tack; pain in side" , "herringbone?" , "One's in time to save eight others" , "something seen in the needlework class" .)

I've seen this clue in the Evening Standard.

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