Without warning, abrupt (6)
I believe the answer is:
sudden
I believe this clue is a double definition.
'without warning' is the first definition.
(I've seen this in another clue)
'abrupt' is the second definition.
(similar in meaning)
(Other definitions for sudden that I've seen before include "Occurring unexpectedly quickly" , "Occurring quickly without warning" , "Rapid and unexpected" , "Happening abruptly" , "Abrupt, unexpected" .)