Article relative needed for dissertation (6)
I believe the answer is:
thesis
'dissertation' is the definition.
('thesis' can be a synonym of 'dissertation')
'article relative needed' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'relative' becomes 'sis' (short for sister).
'needed' means one lot of letters go next to another.
'the'+'sis'='THESIS'
'for' is the link.
(Other definitions for thesis that I've seen before include "Essay putting forward a particular view" , "Researched dissertation" , "Scholarly dissertation" , "student's work" , "Academic dissertation" .)