So happy at first to find tree (3)
I believe the answer is:
Here is my best explanation:
'tree' is the definition.
(ash is a kind of tree)
'so happy at first to find' is the subsidiary indication.
'first' suggests taking the first letters.
'to find' is an anagram indicator.
The first letters of 'so happy at' is 'sha'.
'sha' is an anagram of 'ash'.
(Other definitions for ash that I've seen before include "Burnt matter", "variety", "Burnt material; tree", "Dust", "Grey colour", "Type of tree", "Remains of a fire", "Fire remains", "Dusty residue of a fire" and "Wood".)