Her article is on Monday (4)
I believe the answer is:
mona
'her' is the definition.
'article is on monday' is the wordplay.
'article' becomes 'a' (indefinite article).
'is on' means one lot of letters go next to another.
'monday' becomes 'mon' ('Mon' can be a synonym of 'monday').
'a' after 'mon' is 'MONA'.
(Other definitions for mona that I've seen before include "Female" , "Lisa in the Louvre" .)