Playful behaviour brings end of old association (9)
I believe the answer is:
dalliance
'playful behaviour' is the definition.
'end of old association' is the wordplay.
'end of old' becomes 'd' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'association' becomes 'alliance' (I've seen this before).
'd'+'alliance'='DALLIANCE'
'brings' is the link.
Can you help me to learn more?
(Other definitions for dalliance that I've seen before include "Casual relationship" , "amorous play" , "Toying - trifling - delay" , "waste of time" , "Flirtation" .)