Sound measure? (6)
I believe the answer is:
fathom
I believe this is a double definition.
'sound' is the first definition.
(I know that sound can be written as fathom)
'measure' is the second definition.
(fathom is a kind of measure)
(Other definitions for fathom that I've seen before include "Sound" , "Discover the meaning of - six feet" , "Measure of water depth" , "Unit of measurement for water depth" , "Come to understand - unit of depth" .)