Without warning, unexpectedly (8)
I believe the answer is:
suddenly
I believe this is a double definition.
'without warning' is the first definition.
(I've seen this in another clue)
'unexpectedly' is the second definition.
(I've seen this before)
(Other definitions for suddenly that I've seen before include "Happening without warning" , "Quickly, unexpectedly" , "Out of the blue" , "Quickly and without warning" , "In an unexpected manner" .)