The manuscript is about Eastern subjects (6)
I believe the answer is:
themes
'subjects' is the definition.
(I know this)
'the manuscript is about eastern' is the wordplay.
'manuscript' becomes 'ms' (abbreviation for manuscript).
'is about' indicates putting letters inside.
'eastern' becomes 'e'.
'ms' enclosing 'e' is 'mes'.
'the'+'mes'='THEMES'
(Other definitions for themes that I've seen before include "Subjects or repeated melodies" , "Subjects or repeated musical pieces" , "Subjects or unifying ideas" , "Subject matters, motifs" , "Recurrent ideas" .)