Centre of demonstration in Bow (3)
I believe the answer is:
arc
'bow' is the definition.
(both can mean a curve)
'centre of demonstration' is the wordplay.
'centre of' means to look at the middle letters.
'demonstration' becomes 'march' (type of protest or demonstration).
The central letters of 'march' are 'ARC'.
'in' is the link.
(Other definitions for arc that I've seen before include "Part of a circular line" , "Portion of a circle" , "Semi-circle" , "Segment of circumference" , "be rapidly discharged" .)