Unifying ideas or motifs (6)
I believe the answer is:
themes
This clue is a double definition.
'unifying ideas' is the first definition.
(I've seen this in another clue)
'motifs' is the second definition.
('theme' can be a synonym of 'motif')
(Other definitions for themes that I've seen before include "Topics, ideas, motifs" , "'Subjects, topics (6)'" , "items proposed for discussion" , "Subjects or repeated musical pieces" , "Motifs, subjects" .)