Unrepeated event (3-3)
I believe the answer is:
one-off
'unrepeated event' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for one-off that I've seen before include "Unique person or occurrence" , "Made or done as a single thing, not repeated" , "Item made once only" , "Not repeated or reproduced, a single thing" , "Done a single time" .)