kitty-corner (8)
I believe the answer is:
diagonal
'kitty-corner' is the definition.
(synonyms)
This is all the clue.
(Other definitions for diagonal that I've seen before include "Line from one corner to the fartherst one" , "bishop controls it" , "Line connecting two nonadjacent corners of a rectangle" , "straight line from corner" , "The slanted line between opposite corners" .)