Topic those people start to explore (5)
I believe the answer is:
theme
'topic' is the definition.
(synonyms)
'those people start to explore' is the wordplay.
'those people' becomes 'them' (I've seen this before).
'start to' indicates taking the first letters.
The first letter of 'explore' is 'e'.
'them'+'e'='THEME'
(Other definitions for theme that I've seen before include "Topical item" , "Topic; melody" , "thing under discussion" , "(Writer's) topic" , "sort of tune" .)