Article on extremely mundane subject (5)
I believe the answer is:
theme
'subject' is the definition.
('theme' can be a synonym of 'subject')
'article on extremely mundane' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'on' is a charade indicator (letters next to each other).
'extremely' means to remove the middle letters.
'mundane' with its middle removed is 'me'.
'the'+'me'='THEME'
(Other definitions for theme that I've seen before include "argument" , "Melody on which variations are constructed" , "Subject (of talk, text, etc)" , "issue" , "A subject or repeated melody" .)