Those people taking fifth grade subject (5)
I believe the answer is:
theme
'subject' is the definition.
(theme can mean a topic or subject)
'those people taking fifth grade' is the wordplay.
'those people' becomes 'them' ('they' refers to a group of people).
'taking' is a charade indicator (letters next to each other).
'fifth grade' becomes 'E' (5th letter of 'grade').
'them'+'e'='THEME'
(Other definitions for theme that I've seen before include "Main idea or topic" , "Thread, melody" , "Unifying motif" , "(Writer's) topic" , "Motif that's repeated" .)