It goes round in a circle (3)
I believe the answer is:
This could be a double definition.
'it goes round' is the first definition.
The definition suggests a singular noun which matches the answer.
'circle' is the second definition.
'in a' is the link.
Can you help me to learn more?
(Other definitions for arc that I've seen before include "Car (anag.)" , "Something used to weld" , "Curvature" , "Part of a curve" , "Electric discharge" .)