Measure the depth of (5)
'measure the depth of' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for plumb that I've seen before include "Utterly" , "Test alignment" , "Absolutely straight - sound" , "Lead weight; exactly" , "True" .)
(Other definitions for sound that I've seen before include "strait" , "Measure depth" , "well" , "Valid" , "Health - noise" .)