Proposition is following article on sun (6)
I believe the answer is:
thesis
'proposition' is the definition.
(thesis is a kind of proposition)
'following article on sun' is the wordplay.
'following' is a charade indicator (letters next to each other).
'article' becomes 'the' (the 'definite article' part of speech).
'on' becomes 'si' (I can't explain this - if you can you should believe this answer much more).
'sun' becomes 's'.
'the' put next to 'si' is 'thesi'.
'thesi'+'s'='THESIS'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for thesis that I've seen before include "Learned dissertation" , "Subject" , "Dissertation presented for a doctorate" , "Long research essay or treatise written for university degree" , "He sits (anag) - treatise" .)