Pleasant conversation rules out theft (4,8)
I believe the answer is:
fair exchange
'rules out theft' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'pleasant conversation' is the wordplay.
'pleasant' becomes 'fair' (I am not sure about this - if you are sure you should believe this answer much more).
'conversation' becomes 'exchange' (exchange is a kind of conversation).
'fair'+'exchange'='FAIR EXCHANGE'
Can you help me to learn more?
(Another definition for fair exchange that I've seen is " It's no robbery, they say".)