Perpendicular, upright (8)
I believe the answer is:
vertical
I believe this clue is a double definition.
'perpendicular' is the first definition.
(I know that perpendicular can be written as vertical)
'upright' is the second definition.
(I know that upright can be written as vertical)
(Other definitions for vertical that I've seen before include "Absolutely upright" , "Perpendicular to the horizontal" , "as is column" , "Perfectly upright" , "Ninety degrees to horizontal" .)