Formal piece of writing on a subject (8)
I believe the answer is:
treatise
'formal piece of writing on a subject' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for treatise that I've seen before include "Formal piece of writing on a particular subject" , "Learned work formally dealing with a subject" , "formal document" , "Academic composition" , "Formal written work dealing with a particular subject" .)