Remarkably rapid (8)
I believe the answer is:
meteoric
'remarkably rapid' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for meteoric that I've seen before include "Like a small body in space" , "moving at high speed" , "Dazzling and rapid, rise say" , "(Of change) very rapid" , "Spectacularly fast" .)