Choice by bishop is sound (5)
I believe the answer is:
plumb
'is sound' is the definition.
(both can mean to measure a depth)
'choice by bishop' is the wordplay.
'choice' becomes 'plum' (both can mean excellent).
'by' is a charade indicator (letters next to each other).
'bishop' becomes 'B' (chess abbreviation).
'plum'+'b'='PLUMB'
(Other definitions for plumb that I've seen before include "Dead" , "Get to the bottom of the matter or depths" , "Lead weight for measuring depth" , "Lead weight; exactly" , "Going straight up and down" .)