Friendly mixing is reason rift ain't spreading (14)
I believe the answer is:
fraternisation
'friendly mixing' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe there's a link between them I don't understand?
'reason rift ain't spreading' is the wordplay.
'spreading' indicates an anagram.
'reason'+'rift'+'aint'='reasonriftaint'
'reasonriftaint' with letters rearranged gives 'FRATERNISATION'.
'is' is the link.
Can you help me to learn more?
(Other definitions for fraternisation that I've seen before include "Friendly relationship" , "Getting on with other people" .)