Crossword Genius

Sound line (5)

Ross

I believe the answer is:

plumb

I believe this clue is a double definition.

'sound' is the first definition.
(thesaurus)

'line' is the second definition.
(plumb line is a kind of line)

(Other definitions for plumb that I've seen before include "Exactly - in a vertical line" , "Lead weight; exactly" , "Measure the depth of water" , "Fathom - out-and-out" , "Lead weight for testing depth or the perpendicular" .)

I've seen this clue in The Telegraph.
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