One writing school essay originally without illicit aid (6)
I believe the answer is:
scribe
'one writing school essay originally without illicit aid' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one writing school essay originally without' is the wordplay.
'one writing' becomes 'crib' (I can't justify this - if you can you should believe this answer much more).
'originally' says to take the initial letters.
'without' means one lot of letters goes inside another.
The initial letters of 'school essay' is 'se'.
'crib' placed inside 'se' is 'SCRIBE'.
Can you help me to learn more?
(Other definitions for scribe that I've seen before include "One writing documents" , "Mark with a sharp point" , "A writer of old" , "Medieval clerk" , "A person writing" .)