Subject is at best, impolitic, on the surface (5)
I believe the answer is:
topic
'subject' is the definition.
(as in a topic for discussion)
'at best impolitic on the surface' is the wordplay.
I cannot really understand how this works, but
'best' could be 'top' (top can mean the very best) and 'top' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for topic that I've seen before include "Theme of discourse" , "Subject (eg, of text)" , "Subject (of speech, etc)" , "Matter for an argument" , "Subject matter of a conversation or discussion" .)