Completely sound (5)
I believe the answer is:
plumb
I believe this is a double definition.
'completely' is the first definition.
('plumb' can be a synonym of 'completely')
'sound' is the second definition.
(thesaurus)
(Other definitions for plumb that I've seen before include "Take soundings at sea" , "Vertical; measure (depth)" , "Lead weight; exactly" , "Lead weight for testing depth or the perpendicular" , "Fathom - out-and-out" .)