Significant article about fiction is set in stone (7)
I believe the answer is:
salient
'significant' is the definition.
(thesaurus)
'article about fiction is set in stone' is the wordplay.
'article' becomes 'an' (English indefinite article).
'about' means one lot of letters goes inside another.
'fiction' becomes 'lie' (fiction can mean a lie or fabrication).
'is set in' is an insertion indicator.
'stone' becomes 'st' (abbreviation).
'an' enclosing 'lie' is 'alien'.
'alien' placed into 'st' is 'SALIENT'.
(Other definitions for salient that I've seen before include "part of defences" , "Prominent, most noticeable" , "Projecting outwards; significant" , "Prominent, striking" , "Leaping (heraldry)" .)