Laughing in these, slip, running? (8)
I believe the answer is:
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for stitches that I've seen before include "One of these in time saves nine" , "Sharp pains in the side caused by running" , "As made by 35 across" , "In these, you're laughing hard" , "Sews" .)