Forest, last of it burning (6)
I believe the answer is:
ardent
'burning' is the definition.
(thesaurus)
'forest last of it' is the wordplay.
'forest' becomes 'arden' (I've seen this before).
'last of' says to take the final letters.
The last letter of 'it' is 't'.
'arden'+'t'='ARDENT'
(Other definitions for ardent that I've seen before include "Very keen" , "Enthusiastic or passionate" , "Passionate and glowing" , "passion" , "Burning, passionate" .)