Endless joy in swallow — one born to fly (6)
I believe the answer is:
eaglet
'one born to fly' is the definition.
(young eagle)
'endless joy in swallow' is the wordplay.
'endless' means to remove the last letter.
'joy' becomes 'glee' (I've seen this before).
'in' is an insertion indicator.
'swallow' becomes 'eat' (swallow can mean to consume or eat up).
'glee' with its final letter taken away is 'gle'.
'gle' put into 'eat' is 'EAGLET'.
(Other definitions for eaglet that I've seen before include "A young raptor" , "One hatched in eyrie" , "Those looking lecherously" , "Get ale for the young bird" , "Bird still in nest" .)