A sitter worked on English essay (8)
I believe the answer is:
treatise
'essay' is the definition.
(I've seen this before)
'a sitter worked on english' is the wordplay.
'worked' indicates an anagram.
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'english' becomes 'E' (abbreviation).
'a'+'sitter'='asitter'
'asitter' with letters rearranged gives 'tratise'.
'tratise' enclosing 'e' is 'TREATISE'.
(Other definitions for treatise that I've seen before include "Formal piece of writing on a particular subject" , "Formal written work dealing with a particular subject" , "Formal exposition" , "Learned work formally dealing with a subject" , "A written work about treaties" .)