Part of circle at heart of demo (3)
I believe the answer is:
arc
'part of circle' is the definition.
(part of the circumference of a circle)
'heart of demo' is the wordplay.
'heart of' means to look at the middle letters.
'demo' becomes 'march' (demonstration).
The middle of 'march' is 'ARC'.
'at' is the link.
(Other definitions for arc that I've seen before include "Bow" , "bends?" , "be rapidly discharged" , "Curved section" , "circle, partly" .)