Sad secret disturbed philosopher (9)
I believe the answer is:
descartes
'philosopher' is the definition.
(I know that Descartes is a philosopher)
'sad secret disturbed' is the wordplay.
'disturbed' indicates anagramming the letters (the order of letters is disturbed).
'sad'+'secret'='sadsecret'
'sadsecret' with letters rearranged gives 'DESCARTES'.
(Other definitions for descartes that I've seen before include "Cogito, ergo sum man" , "Cogito ergo sum philosopher" , "French mathematician and philosopher" , "French philosopher, d. 1650" , "One who thought" .)