Bertrand we hear making a sound (6)
I believe the answer is:
rustle
'a sound' is the definition.
(I know that rustle is a sound)
'bertrand we hear' is the wordplay.
'bertrand' becomes 'Russell'.
'we hear' indicates a 'sounds like' (homophone) clue.
'russell' is a homophone of 'RUSTLE'.
'making' acts as a link.
(Other definitions for rustle that I've seen before include "take animals" , "Steal (cattle, etc)" , "Steal cattle or livestock with whispering sound" , "Steal 2" , "Whisper softly" .)