At last Greta will be alone in this truly sylvan place. (8)
I believe the answer is:
arboreal
'at last' is the definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I don't see?
'greta will be alone in this truly sylvan place' is the wordplay.
I cannot really understand how this works, but
'truly' could be 'real' (synonyms) and 'real' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for arboreal that I've seen before include "found in 20 [TREETOP] ?" , "Like the sloth" , "Tree-dwelling" , "living in forest" , "Relating to trees" .)