Those people initially establish unifying idea (5)
I believe the answer is:
theme
'unifying idea' is the definition.
(I've seen this before)
'those people initially establish' is the wordplay.
'those people' becomes 'them' ('they' refers to a group of people).
'initially' says to take the initial letters.
The initial letter of 'establish' is 'e'.
'them'+'e'='THEME'
(Other definitions for theme that I've seen before include "subject,motif" , "Subject (of talk, text, etc)" , "Thread, melody" , "Subject; recurring idea" , "Motif or melody" .)