Odd trees containing good birds (6)
I believe the answer is:
egrets
'birds' is the definition.
(egret is a kind of bird)
'odd trees containing good' is the wordplay.
'odd' indicates an anagram.
'containing' means one lot of letters goes inside another.
'good' becomes 'g' (abbreviation).
'trees' with letters rearranged gives 'erets'.
'erets' placed around 'g' is 'EGRETS'.
(Other definitions for egrets that I've seen before include "Fliers" , "Greets like fishing eagles" , "Greets like certain herons" , "White wading birds" , "waders" .)