Article relative to treatise (6)
I believe the answer is:
thesis
'to treatise' is the definition.
(thesis is a kind of treatise)
'article relative' is the wordplay.
'article' becomes 'the' (the definite article in English).
'relative' becomes 'sis' (short for sister).
'the'+'sis'='THESIS'
(Other definitions for thesis that I've seen before include "Subject" , "Lengthy written essay for university degree" , "Dissertation - idea" , "Treatise advancing a new point of view resulting from research" , "Academic essay" .)