Descartes declaring himself the personification of peace (5)
I believe the answer is:
irene
'personification of peace' is the definition.
(I've seen this in another clue)
'descartes declaring himself' is the wordplay.
I cannot quite see how this works, but
'descartes' could be 'rene' (as in Rene Descartes the French philosopher) and 'rene' is present in the answer.
A single letter 'i' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Other definitions for irene that I've seen before include "Greek godess of peace" , "Female name" , "One can find a girl in Ree" , "Girl of peace" , "- - Handl, actress" .)