Vertical, perpendicular (5)
I believe the answer is:
plumb
I believe this is a double definition.
'vertical' is the first definition.
(similar in meaning)
'perpendicular' is the second definition.
(I've seen this in another clue)
(Other definitions for plumb that I've seen before include "Absolutely straight - sound" , "Lead weight; exactly" , "Check depth of water below" , "Fathom - out-and-out" , "Going straight up and down" .)