Dissertation not say in essay, described with what we have here (6)
I believe the answer is:
thesis
This clue could be a double definition.
'dissertation' is the first definition.
(synonyms)
'not say in essay described with what we have here' is the second definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
(Other definitions for thesis that I've seen before include "He sits (anag) - treatise" , "Essay putting forward a particular view" , "Scholarly dissertation" , "Proposition" , "Student dissertation" .)