Tune; topic (5)
I believe the answer is:
theme
This clue is a double definition.
'tune' is the first definition.
(I know that theme is a type of tune)
'topic' is the second definition.
(I know that topic can be written as theme)
(Other definitions for theme that I've seen before include "Motif, topic" , "Main idea or topic" , "tune used in Enigma Variations?" , "Topic for discussion" , "argument" .)