Subject of note crossing border (5)
I believe the answer is:
theme
'subject' is the definition.
('theme' can be a synonym of 'subject')
'note crossing border' is the wordplay.
'note' becomes 'te' (**).
'crossing' means one lot of letters goes inside another.
'border' becomes 'hem' (I've seen this before).
'te' going around 'hem' is 'THEME'.
'of' acts as a link.
(Other definitions for theme that I've seen before include "Topic, leitmotif" , "Topical item" , "Topic; melody" , "matter for discussion" , "Topic for discussion" .)