Philosopher's whisper is audible (7)
I believe the answer is:
russell
'philosopher's' is the definition.
(Bertrand Russell)
'whisper is audible' is the wordplay.
'whisper' becomes 'rustle' (synonyms).
'is audible' indicates a 'sounds like' (homophone) clue.
'rustle' is a homophone of 'RUSSELL'.
(Other definitions for russell that I've seen before include "Ken -, Eng. film director (Women in Love)" , "See 2" , "Jane -, US actress" , "He controversially filmed" , "actress (Rosalind or Jane)" .)