Underlying motif (5)
I believe the answer is:
theme
'underlying motif' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for theme that I've seen before include "He met (anag) - subject" , "Main idea or topic" , "Motif or melody" , "Topic, leitmotif" , "Motif, topic" .)