On which to have a heated conversation? (7)
I believe the answer is:
hotline
'on which to have a' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'heated conversation?' is the wordplay.
'heated' becomes 'hot' (similar in meaning).
'conversation?' becomes 'line' (I can't justify this - if you can you should believe this answer much more).
'hot'+'line'='HOTLINE'
Can you help me to learn more?
(Other definitions for hotline that I've seen before include "Direct telephone link between two officials" , "critical link" , "linked Washington to the Kremlin" , "Direct phone access (for presidents?)" , "Direct phone link" .)