This may stop you running to join up (6)
I believe the answer is:
stitch
'join up' is the definition.
'stitch' can be an answer for 'join' (thesaurus). I am unsure of the 'up' bit.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for stitch that I've seen before include "Application of needle produces this" , "One's in time to save eight others" , "Link, loop or knot made in knitting or sewing" , "result of working with needle?" , "herringbone?" .)