Striking very fast (8)
I believe the answer is:
cracking
This clue is a double definition.
'striking' is the first definition.
(both can mean very good)
'very fast' is the second definition.
(I've seen this before)
(Other definitions for cracking that I've seen before include "failing" , "Solving" , "very fast" , "Wonderful" , "Excellent; fracturing" .)