Fly when very young? (6)
I believe the answer is:
maggot
'fly' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for maggot that I've seen before include "Legless larva" , "Larva often found in decaying matter" , "youngster to fly" , "Larva of housefly or bluebottle" , "immature flyer" .)