Happening without warning (6)
I believe the answer is:
sudden
'happening without warning' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sudden that I've seen before include "'Unexpectedly, without warning (6)'" , "Happening quickly and without warning" , "Occurring quickly without warning" , "Abrupt, unexpected" , "Happening abruptly" .)