They’re peripheral (5)
I believe the answer is:
edges
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 edges that I've seen before include "Boundaries of a surface" , "Des, e.g. (anag.)" , "They're farthest from the middle" , "Borders, rims" , "Sharpened sides of knives etc" .)