Those people ultimately rule subject (5)
I believe the answer is:
theme
'subject' is the definition.
('theme' can be a synonym of 'subject')
'those people ultimately rule' is the wordplay.
'those people' becomes 'them' (I've seen this before).
'ultimately' says to take the final letters.
The final letter of 'rule' is 'e'.
'them'+'e'='THEME'
(Other definitions for theme that I've seen before include "Recurrent unifying idea" , "Leitmotiv" , "Topic; melody" , "Unifying motif" , "sort of tune" .)