Speedy fellow delivering a batch of Whoppers (6)
I believe the answer is:
flying
'speedy' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for flying that I've seen before include "-- Buttress; -- doctor" , "short" , "In the air; hurrying" , "-- boat; -- buttress" , "Taking to the air" .)