Sudden rush (8)
I believe the answer is:
stampede
'sudden rush' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stampede that I've seen before include "Rapid flight" , "herd scatter wildly" , "Panic-stricken charge of cattle" , "Headlong rush of a group in panic" , "Deems apt to have flight of cattle" .)