Crossword Genius

Bob depends on this sound policy (5,4)

Ross

I believe the answer is:

plumb line

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'bob depends on this sound policy' is the definition.
'plumb-line' can be an answer for 'sound' (thesaurus). I'm not sure about the rest of the definition.

'on this sound policy' is the wordplay.
'on' says to put letters next to each other.
'this sound' becomes 'plumb' (both can mean to measure a depth).
'policy' becomes 'line' (as in the party line).
'plumb' next to 'line' is 'PLUMB-LINE'.

Can you help me to learn more?

(Other definitions for plumb line that I've seen before include "Device to show the vertical" , "Verticality checker" , "it hangs straight down" , "This shows perpendicular" .)

I've seen this clue in the Sydney Morning Herald.
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