He thought he was mad, asserted leader of council (9)
I believe the answer is:
descartes
'he thought he' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'mad asserted leader of council' is the wordplay.
'mad' indicates an anagram.
'leader of' suggests taking the first letters.
The first letter of 'council' is 'c'.
'asserted'+'c'='assertedc'
'assertedc' with letters rearranged gives 'DESCARTES'.
'was' acts as a link.
Can you help me to learn more?
(Other definitions for descartes that I've seen before include "Cogito ergo sum philosopher" , "Cogito, ergo sum man" , "French mathematician and philosopher" , "Rene --, philosopher" , "One who thought" .)