Set back about border subjects (6)
I believe the answer is:
themes
'subjects' is the definition.
(I know this)
'set back about border' is the wordplay.
'back' shows that the letters should be reversed in order.
'about' indicates putting letters inside.
'border' becomes 'hem' (I've seen this before).
'set' back-to-front is 'tes'.
'tes' going around 'hem' is 'THEMES'.
(Other definitions for themes that I've seen before include "Subject matters, motifs" , "Topics, ideas, motifs" , "Subjects or unifying ideas" , "'Subjects, topics (6)'" , "Motifs, subjects" .)