Unrepeated event (3-3)
'unrepeated event' is the definition.
(I've seen this in another clue)
This is the entire 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" .)
(Other definitions for oneoff that I've seen before include "Single" , "not repeated" , "Unique" .)