Unabashedly heartened by tree (3)
I believe the answer is:
ash
'tree' is the definition.
(ash is a kind of tree)
'unabashedly heartened' is the wordplay.
'ASH' is hidden within the letters.
But, I'm not sure how this is indicated.
'by' is the link.
Can you help me to learn more?
(Other definitions for ash that I've seen before include "Grey colour" , "Grey-barked tree" , "Powdery residue from fire or cigarette" , "Get the residue" , "What's often left after something 1 Ac" .)