Some travel guides in his language for philosopher (9)
I believe the answer is:
descartes
'philosopher' is the definition.
(I know this)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for descartes that I've seen before include "Cogito ergo sum philosopher" , "One who thought" , "French philosopher, d. 1650" , "French mathematician and philosopher" , "Rene --, philosopher" .)