Unexpected (6)
'unexpected' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for sudden that I've seen before include "Rapid and unexpected" , "Quick and unexpected" , "Swift" , "'Unexpectedly, without warning (6)'" , "Occurring unexpectedly quickly" .)
(Other definitions for untoward that I've seen before include "Inopportune" , "Improper" , "Drawn out to be inconvenient or awkward" , "Inappropriate, undesirable" , "Unseemly" .)
'unexpected' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for surprise that I've seen before include "Shock" , "Startle, astonish" , "Sudden astounding event" , "Sudden unexpected event" , "Take unawares" .)