Unifying idea (5)
I believe the answer is:
theme
'unifying idea' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for theme that I've seen before include "Subject (of talk, text, etc)" , "Topic, leitmotif" , "idea of the work" , "Thread, melody" , "[(type of) PARK]" .)