Academic essay (5)
'academic essay' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for thesis that I've seen before include "Academic dissertation" , "A lengthy essay, perhaps for degree" , "Long dissertation written to obtain university degree" , "Theme for an essay" , "writing following research" .)
(Other definitions for paper that I've seen before include "conference presentation" , "News medium" , "Literary essay" , "Material made from wood pulp, say" , "See 10/25/18/20" .)