Forest with walnut ultimately burning (6)
I believe the answer is:
ardent
'burning' is the definition.
(both can mean passionate)
'forest with walnut ultimately' is the wordplay.
'forest' becomes 'arden' (Forest of Arden).
'with' is a charade indicator (letters next to each other).
'ultimately' says to take the final letters.
The last letter of 'walnut' is 't'.
'arden'+'t'='ARDENT'
(Other definitions for ardent that I've seen before include "Passionate, fervent" , "Warm and loving" , "Intensely devoted" , "Loving and fiery" , "Passionate, full of feeling" .)