Philosopher extremely disagreeable actress upset (9)
I believe the answer is:
descartes
'philosopher' is the definition.
(Descartes is an example)
'extremely disagreeable actress upset' is the wordplay.
'extremely' means to remove the middle letters.
'upset' is an anagram indicator.
'disagreeable' with its middle taken out is 'de'.
'actress' anagrammed gives 'scartes'.
'de'+'scartes'='DESCARTES'
(Other definitions for descartes that I've seen before include "One who thought" , "French philosopher, d. 1650" , "Cogito, ergo sum man" , "Cogito ergo sum philosopher" , "French mathematician and philosopher" .)