From corner to corner (8)
I believe the answer is:
diagonal
'from corner to corner' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for diagonal that I've seen before include "Slanting line joining two opposite corners" , "bishop controls it" , "Corner-to-corner line" , "Sloping line" , "Line from one corner to the opposite corner" .)