Those people originally explained topic (5)
I believe the answer is:
theme
'topic' is the definition.
(synonyms)
'those people originally explained' is the wordplay.
'those people' becomes 'them' ('they' refers to a group of people).
'originally' suggests taking the first letters.
The initial letter of 'explained' is 'e'.
'them'+'e'='THEME'
(Other definitions for theme that I've seen before include "Short melody" , "Subject set for discussion" , "Motif, topic" , "Musical idea; topic" , "Subject; recurring idea" .)