Moving rapidly (4)
(Other definitions for fast that I've seen before include "Secure; speedy" , "Go without food" , "Period without food" , "Rapid; abstain from food" , "Quick; firmly fixed" .)
'moving rapidly' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for flying that I've seen before include "& 20 Maritime aircraft" , "Moving quickly" , "-- boat; -- buttress" , "''Those magnificent men in their ... machines'' (film theme song)" , "Travelling by plane" .)