Quickly and without warning (8)
I believe the answer is:
abruptly
'quickly and without warning' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for abruptly that I've seen before include "brusque manner" , "In curt or disconnected manner" , "Quickly and without warning" , "In a sudden way" , "Suddenly and unexpectedly" .)