Sudden, unexpected (6)
I believe the answer is:
abrupt
This clue is a double definition.
'sudden' is the first definition.
(similar in meaning)
'unexpected' is the second definition.
(I've seen this in another clue)
(Other definitions for abrupt that I've seen before include "Unexpected -- precipitous" , "Offhand" , "Short in manner" , "Sudden, maybe discourteous" , "Brusque, curt" .)