Subject, topic (5)
I believe the answer is:
theme
I believe this clue is a double definition.
'subject' is the first definition.
(theme can mean a topic or subject)
'topic' is the second definition.
(I know that topic can be written as theme)
(Other definitions for theme that I've seen before include "Musical motif" , "Unifying motif" , "Motif - 4 across" , "notedly subject to variation" , "He met (anag) - subject" .)