Topic for debate is petition (5)
I believe the answer is:
issue
'topic for debate' is the definition.
(I've seen this before)
'is petition' is the wordplay.
'petition' becomes 'sue' (I've seen this in other clues).
'is'+'sue'='ISSUE'
(Other definitions for issue that I've seen before include "important subject" , "Subject for discussion; dish out" , "Question in dispute to be settled" , "Shares" , "Put out; topic for discussion" .)