Study showing doctor is English (8)
I believe the answer is:
treatise
'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).
'treat'+'is'+'e'='TREATISE'
'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" .)