Swift – doing what it does? (6)
I believe the answer is:
flying
'swift' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for flying that I've seen before include "Taking flight" , "Moving quickly" , "Famous train once from King's Cross" , "Moving rapidly; brief" , "UFO" .)