Completely vertical (5)
I believe the answer is:
plumb
I believe this is a double definition.
'completely' is the first definition.
(synonyms)
'vertical' is the second definition.
(similar in meaning)
(Other definitions for plumb that I've seen before include "Connect pipes" , "True" , "Fathom - out-and-out" , "Test alignment" , "Precisely" .)