Final character in set troubled revolutionary philosopher (9)
I believe the answer is:
nietzsche
'philosopher' is the definition.
(I've seen this before)
'final character in set troubled revolutionary' is the wordplay.
I cannot quite see how this works, but
'character' could be 'he' (he is a kind of letter of the alphabet) and 'he' is present in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for nietzsche that I've seen before include "German philosopher, wrote Beyond Good and Evil" , "19th century German philosopher who divided mankind into master class and herd" , "Friedrich -, nineteenth century German philosopher" , "German philosopher, d. 1900, noted for his concept of superman" , "Influential German existentialist philosopher" .)