Playful behaviour brings end of old association (9)
I believe the answer is:
Here is my best explanation:
'playful behaviour' is the definition.
'end of old association' is the subsidiary indication.
'end of' says to take the final letters.
'association' becomes 'alliance' (I've seen this before).
The final letter of 'old' is 'd'.
'brings' is the link.
(Other definitions for dalliance that I've seen before include "Toying - trifling - delay", "Flirtation", "affair", "Fling", "Playing", "Trifling", "Affair", "Delay", "Casual relationship" and "time-wasting action".)