Brand may be heard making a sound (6)
I believe the answer is:
rustle
'a sound' is the definition.
(I know that rustle is a sound)
'brand may be heard' is the wordplay.
'brand' becomes 'russell' (I've seen this before).
'may be heard' indicates a 'sounds like' (homophone) clue.
'russell' sounds like 'RUSTLE'.
'making' acts as a link.
(Other definitions for rustle that I've seen before include "Steal 2" , "Steal livestock - make a little noise" , "Steal stock" , "take animals" , "Swish - steal cattle" .)