Without warning (8)
There are a few possibilities for this:
suddenly
'without warning' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for suddenly that I've seen before include "Quickly, unexpectedly" , "In an unexpected manner" , "Unexpectedly, abruptly" , "Without any warning, unexpectedly" , "Quickly and without warning" .)
'without warning' is the definition.
(thesaurus)
This is all the clue.
(Another definition for unannounced that I've seen is " Not publicised".)