The best piece of fiction is apt to be discussed (5)
I believe the answer is:
topic
'fiction is apt to be discussed' is the definition.
I can't tell whether this defines the answer.
'the best piece' is the wordplay.
'the best' becomes 'top' (top can mean the very best).
'piece' becomes 'ic' (I can't justify this - if you can you should believe this answer much more).
'top'+'ic'='TOPIC'
'of' is the link.
Can you help me to learn more?
(Other definitions for topic that I've seen before include "Subject matter, theme" , "Something to discuss" , "Theme of discourse" , "Subject (of a talk, etc)" , "That may be debatable" .)