Going by air is loud but not heavy (6)
I believe the answer is:
flight
'going' is the definition.
(thesaurus)
'air is loud but not heavy' is the wordplay.
I cannot really see how this works, but
'loud' could be 'f' (short for forte - 'loud' in music) and 'f' is found in the answer.
'not heavy' could be 'light' ('light' is the opposite to 'heavy') and 'light' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'by' acts as a link.
Can you help me to learn more?
(Other definitions for flight that I've seen before include "Exodus" , "Stairs; escape" , "Series of steps" , "A series of stairs" , "Air force unit; running away" .)