Move swiftly, as clouds (4)
I believe the answer is:
scud
'move swiftly as clouds' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for scud that I've seen before include "Gust of wind" , "(Of clouds) move quickly" , "Sail swiftly before the wind - missile" , "Ballistic missile; sweep along" , "Stark in Banff" .)