Philosopher speaking in a crackly whisper (6)
I believe the answer is:
rustle
'a crackly whisper' is the definition.
(rustle can mean a whispering sound)
'philosopher speaking' is the wordplay.
'philosopher' becomes 'Russell' (Bertrand Russell).
'speaking' shows a homophone (sound like).
'russell' is a homophone of 'RUSTLE'.
'in' acts as a link.
(Other definitions for rustle that I've seen before include "Steal cattle in the old West" , "Swish - steal cattle" , "Steal livestock - make a little noise" , "The sound of leaves in the wind" , "Result (anag.)" .)