Late-developing duck (4)
I believe the answer is:
teal
'duck' is the definition.
(teal is a kind of duck)
'late-developing' is the wordplay.
'developing' is an anagram indicator.
'late' is an anagram of 'TEAL'.
(Other definitions for teal that I've seen before include "bird" , "Small freshwater duck and colour" , "Dark green-blue; duck" , "Duck; greenish-blue colour" , "Species of duck" .)