Sound way to use a balance (5)
I believe the answer is:
weigh
'a balance' is the definition.
(I've seen this before)
'sound way' is the wordplay.
'sound' indicates a 'sounds like' (homophone) clue.
'way' is a homophone of 'WEIGH'.
'to use' acts as a link.
(Other definitions for weigh that I've seen before include "Measure heaviness" , "Find out how heavy (something) is" , "Bear down oppressively" , "Evaluate (in mind)" , "Put in the balance?" .)