Philosopher sad, secret out (9)
I believe the answer is:
descartes
'philosopher' is the definition.
(Descartes is an example)
'sad secret out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'sad'+'secret'='sadsecret'
'sadsecret' is an anagram of 'DESCARTES'.
(Other definitions for descartes that I've seen before include "French mathematician and philosopher" , "French philosopher, d. 1650" , "Cogito ergo sum philosopher" , "One who thought" , "Cogito, ergo sum man" .)