Study agreements head of state promoted (8)
I believe the answer is:
treatise
'study' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for treatise that I've seen before include "Academic composition" , "Formal written work dealing with a particular subject" , "Tract, monograph" , "Formal exposition" , "Iterates in dissertation" .)