Partly Eastern bird (4)
I believe the answer is:
tern
'partly eastern bird' is the definition.
'tern' can be an answer for 'bird' (tern is a kind of bird). I am not sure about the 'partly eastern' bit.
'partly eastern bird' is the wordplay.
'partly' indicates the answer is hidden within the clue (part of the clue is used).
'TERN' is hidden in the letters of 'eastern bird'.
Can you help me to learn more?
(Other definitions for tern that I've seen before include "Seabird, sounds like become sour" , "Three-masted schooner" , "Fork-tailed seabird" , "Winger" , "Pay rent for seabird" .)