It's up for debate (6)
(Other definitions for motion that I've seen before include "Flow - proposal" , "inducing nausea in some travellers" , "Proposal for action" , "Proposal for discussion or a vote" , "MP's proposal" .)
'it's up for debate' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for topic that I've seen before include "Question" , "Content of argument" , "Subject (of a talk, etc)" , "Issue (for discussion)" , "Talking point" .)