Issue to discuss (5)
I believe the answer is:
'issue to discuss' 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 "A subject for conversation or debate" , "Subject (of a talk, etc)" , "Subject (of speech, etc)" , "Issue (for discussion)" , "Subject (eg, of text)" .)