So happy at first to find tree (3)
I believe the answer is:
ash
'tree' is the definition.
(ash tree is a kind of tree)
'so happy at first to find' is the wordplay.
'first' says to take the initial letters.
'to find' indicates anagramming the letters.
The initial letters of 'so happy at' is 'sha'.
'sha' is an anagram of 'ASH'.
(Other definitions for ash that I've seen before include "Remains of conflagration" , "Tree; burnt residue" , "Fire's remains" , "End of fire" , "Silvery grey" .)