They're notedly subject to variations (6)
I believe the answer is:
themes
'they're notedly' is the definition.
The definition suggests an adverb but the answer is not.
'subject to variations' is the wordplay.
'subject' becomes 'theme' (theme can mean a topic or subject).
'to variations' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'theme'+'s'='THEMES'
Can you help me to learn more?
(Other definitions for themes that I've seen before include "Subjects or repeated musical pieces" , "Subjects or unifying ideas" , "Motifs, subjects" , "Subject matters, motifs" , "Topics, ideas, motifs" .)