Article has limited space for subject (5)
I believe the answer is:
theme
'subject' is the definition.
(theme can mean a topic or subject)
'article has limited space' is the wordplay.
'article' becomes 'the' (the definite article).
'has' indicates putting letters inside.
'limited space' becomes 'em' (in typography, a space as wide as a letter 'm'. I am not sure about the 'limited' bit.).
'the' placed around 'em' is 'THEME'.
'for' is the link.
Can you help me to learn more?
(Other definitions for theme that I've seen before include "Thread, melody" , "thing under discussion" , "(Writer's) topic" , "Topic; melody" , "matter for discussion" .)