Article is about singular line of argument (6)
I believe the answer is:
thesis
'argument' is the definition.
(I've seen this in another clue)
'article is about singular line' is the wordplay.
'article' becomes 'the' (the definite article).
'about singular' becomes 's' (abbreviation in grammar. I am not sure about the 'about' bit.).
'line' is an insertion indicator.
'the'+'is'='theis'
'theis' placed around 's' is 'THESIS'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for thesis that I've seen before include "He sits (anag) - treatise" , "Academic dissertation" , "Research essay written to obtain university degree" , "View advanced for purposes of argument" , "Dissertation presented for a doctorate" .)