Ball - possibly square (5)
I believe the answer is:
dance
This could be a double definition.
'ball possibly' is the first definition.
(ball is a kind of dance)
'square' is the second definition.
The definition and answer can be both to do with motion as well as being verbs in their base form.
Perhaps you can see a link between them that I can't see?
Can you help me to learn more?
(Other definitions for dance that I've seen before include "Waltz, say" , "Eg, perform ballet" , "Eg, foxtrot or tango" , "Perhaps reel" , "social event" .)