Surprise! After brief wait trifle, if dropped, reconstituted (7)
I believe the answer is:
startle
'surprise' is the definition.
(to startle someone is to surprise or shock them)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for startle that I've seen before include "Frighten suddenly" , "Rock" , "Alarm suddenly" , "Make jump" , "Cause to feel sudden alarm" .)