Subject of article about space (5)
I believe the answer is:
theme
'subject' is the definition.
(synonyms)
'article about space' is the wordplay.
'article' becomes 'the' (the definite article in English).
'about' is a reversal indicator.
'space' becomes 'em' (in typography, a space as wide as a letter 'm').
'em' reversed gives 'me'.
'the'+'me'='THEME'
'of' is the link.
(Other definitions for theme that I've seen before include "Subject; recurring idea" , "Musical idea; topic" , "subject,motif" , "Leitmotiv" , "Signature tune" .)