Subject matter (5)
I believe the answer is:
theme
'subject matter' is the definition.
(I know that theme is a type of subject matter)
This is the entire clue.
(Other definitions for theme that I've seen before include "Subject; recurring idea" , "Thread, melody" , "(Writer's) topic" , "Subject of a talk or piece of writing" , "Motif - 4 across" .)