Study showing doctor is English (8)
I believe the answer is:
'study' is the definition.
(I've seen this before)
'doctor is english' is the wordplay.
'doctor' becomes 'treat' (I've seen this before).
'english' becomes 'e' (abbreviation).
'showing' is the link.
(Other definitions for treatise that I've seen before include "Long, detailed essay" , "Learned work formally dealing with a subject" , "Written work dealing with one subject" , "Formal exposition" , "Learned writing" .)