Line connecting two nonadjacent corners (8)
I believe the answer is:
diagonal
'line connecting two nonadjacent corners' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for diagonal that I've seen before include "Slanting line joining two opposite corners" , "Line connecting two nonadjacent corners of a rectangle" , "It joins square's opposite corners" , "Slanting, from corner to corner" , "Corner-to-corner line" .)