Recurrent idea or subject (5)
I believe the answer is:
theme
I believe this clue is a double definition.
'recurrent idea' is the first definition.
(I've seen this before)
'subject' is the second definition.
(theme can mean a topic or subject)
(Other definitions for theme that I've seen before include "Signature tune" , "Musical idea; topic" , "(Writer's) topic" , "Motif, topic" , "Thread, melody" .)