Immature flyer's city rate (5)
I believe the answer is:
nymph
'immature flyer's' is the definition.
(immature insect)
'city rate' is the wordplay.
'city' becomes 'ny' (short for New York).
'rate' becomes 'mph' (miles per hour).
'ny'+'mph'='NYMPH'
(Other definitions for nymph that I've seen before include "Gorgeous girl" , "Young dragonfly" , "Beautiful female" , "Mythical spirit maiden; dragonfly life stage" , "Young locust" .)