Talk for a change about heartless agent (12)
I believe the answer is:
conversation
'talk' is the definition.
(I've seen this before)
'change about heartless agent' is the wordplay.
'change' becomes 'conversion' (conversion is a kind of change).
'about' is an insertion indicator.
'heartless agent' becomes 'at' (I can't explain this - if you can you should give a lot more credence to this answer).
'conversion' going around 'at' is 'CONVERSATION'.
'for a' is the link.
Can you help me to learn more?
(Other definitions for conversation that I've seen before include "Talk, chat" , "dialogue" , "22 between people" , "Spoken exchange" , "discussion" .)