Young flier upset about broken leg (6)
I believe the answer is:
eaglet
'young flier' is the definition.
(young eagle)
'upset about broken leg' is the wordplay.
'upset' becomes 'eat' (eat can mean to upset or distress).
'about' indicates putting letters inside.
'broken' indicates an anagram.
'leg' anagrammed gives 'gle'.
'eat' placed around 'gle' is 'EAGLET'.
(Other definitions for eaglet that I've seen before include "Bird still in nest" , "A young raptor" , "Fledgling" , "another of the golden generation?" , "Raptor chick" .)