Going by air is pretty quick (5)
I believe the answer is:
swift
'quick' is the definition.
(I've seen this in another clue)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for swift that I've seen before include "Anglo-Irish satirist" , "Fast-moving bird" , "like a peregrine falcon?" , "flying" , "Jonathan . . . . . wrote 'Gulliver's Travels'" .)