Endless joy seen in swallow? One born to fly (6)
I believe the answer is:
eaglet
'one born to fly' is the definition.
(young eagle)
'endless joy seen in swallow?' is the wordplay.
'endless' means to remove the last letter.
'joy' becomes 'glee' (I've seen this in another clue).
'seen in' indicates putting letters inside.
'swallow?' becomes 'eat' (swallow can mean to consume or eat up).
'glee' with its last letter taken away is 'gle'.
'gle' put within 'eat' is 'EAGLET'.
(Other definitions for eaglet that I've seen before include "One hatched in eyrie" , "Bird still in nest" , "A young raptor" , "Little bird" , "Those looking lecherously" .)