Idea expanded in article by yours truly (5)
I believe the answer is:
theme
'idea expanded' is the definition.
(idea expanded on in a piece of writing)
'article by yours truly' is the wordplay.
'article' becomes 'the' (the definite article in English).
'by' says to put letters next to each other.
'yours truly' becomes 'me' (the person that set this crossword).
'the'+'me'='THEME'
'in' acts as a link.
(Other definitions for theme that I've seen before include "(Writer's) topic" , "Recurrent unifying idea" , "Musical motif" , "Subject set for discussion" , "essay" .)