Young bird, not yet fledged (8)
I believe the answer is:
nestling
'young bird not yet fledged' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nestling that I've seen before include "Young bird - snuggling" , "not yet put out" , "Young bird not yet fledged" , "Unfledged bird" , "getting under duvet?" .)