Move very fast (6)
I believe the answer is:
streak
'move very fast' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for streak that I've seen before include "Long mark, run swiftly" , "Unbroken series, of luck say" , "Move very fast; thin strip" , "Long thin mark" , "Run naked in a public place" .)