Treatise on Princess's route (9)
I believe the answer is:
discourse
'treatise' is the definition.
(thesaurus)
'on princess's route' is the wordplay.
'on' says to put letters next to each other.
'princess's' becomes 'Di's' (Princess Diana).
'route' becomes 'course' (course is a kind of route).
'dis'+'course' is 'DISCOURSE'.
(Other definitions for discourse that I've seen before include "To hold a conversation" , "Conversation, dialogue" , "Speech - conversation" , "Engage in conversation; debate" , "Formal discussion of a topic" .)