Overly self-satisfied trees (4)
I believe the answer is:
gums
'trees' is the definition.
(gum is a kind of tree)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for gums that I've seen before include "Glues" , "Teeth are set in them" , "Trees" , "Smug around the mouth" .)