Ready to fly, female cuckoo gleefully did -- one's taken off (5-7)

'ready to fly' is the definition.
(with fully developed feathers)

'female cuckoo gleefully did one's taken off' is the wordplay.
'female' becomes 'f' (common abbreviation).
'cuckoo' indicates anagramming the letters (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'taken off' indicates named letters should be taken away.
'gleefully'+'did'='gleefullydid'
'gleefullydid' with 'i' taken away is 'gleefullydd'.
'gleefullydd' anagrammed gives 'ullyfledged'.
'f'+'ullyfledged'='FULLY-FLEDGED'