Subject of article — Simeon's heart (5)
I believe the answer is:
theme
'subject' is the definition.
(theme can mean a topic or subject)
'article simeon's heart' is the wordplay.
'article' becomes 'the' (the definite article in English).
'heart' means to look at the middle letters.
The middle of 'simeon' is 'me'.
'the'+'me'='THEME'
'of' acts as a link.
(Other definitions for theme that I've seen before include "Subject matter, topic" , "Musical motif" , "Burden" , "notedly subject to variation" , "Topic for discussion" .)