Once wooded region, forest ultimately burning (6)
I believe the answer is:
ardent
'burning' is the definition.
(both can mean passionate)
'once wooded region forest ultimately' is the wordplay.
'once wooded region' becomes 'Arden' (Forest of Arden, historical forest).
'ultimately' indicates one should take the final letters.
The final letter of 'forest' is 't'.
'arden'+'t'='ARDENT'
(Other definitions for ardent that I've seen before include "very interested" , "full of zeal" , "Passionate, fervent" , "Ranted, being impassioned" , "Enthusiastic or passionate" .)