Caught and gently treated missing large young bird (6)
I believe the answer is:
cygnet
'young bird' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for cygnet that I've seen before include "Young aquatic bird" , "Immature swan" , "Sounds like the type of ring" , "Young water bird" , "Youthful swan" .)