Cube's dozen (5)
I believe the answer is:
edges
'cube's dozen' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for edges that I've seen before include "Boundaries of a surface" , "Outer limits" , "Sharpened sides of knives etc" , "Bounds" , "Sharpened sides of blades" .)