Source of pleasure is English essay (8)
I believe the answer is:
treatise
'essay' is the definition.
(I've seen this before)
'source of pleasure is english' is the wordplay.
'source of pleasure' becomes 'treat' (I've seen this before).
'english' becomes 'e' (abbreviation).
'treat'+'is'+'e'='TREATISE'
(Other definitions for treatise that I've seen before include "Learned written work" , "Learned paper" , "Formal written work dealing with a particular subject" , "Long, detailed essay" , "Written work dealing with one subject" .)