Out of the blue (8)
I believe the answer is:
suddenly
'out of the blue' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for suddenly that I've seen before include "Without any warning, unexpectedly" , "Unexpectedly, abruptly" , "Quickly, unexpectedly" , "Out of the blue" , "Happening without warning" .)