Lecture about how birds preen themselves (5,4)
I believe the answer is:
dress down
'lecture' is the definition.
(I've seen this before)
'preen themselves' is the wordplay.
'preen' becomes 'dress' (synonyms).
'themselves' becomes 'down' (I can't explain this - if you can you should give a lot more credence to this answer).
'dress'+'down'='DRESS DOWN'
'about how birds' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for dress down that I've seen before include "Slob around" , "wig" , "Maybe keep tie off" , "Scold - don't wear best clothes" , "Sends word to give scolding to" .)