Where Tree mostly would inspire enthusiasm! (7)
I believe the answer is:
theatre
'where tree mostly would inspire enthusiasm' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'where tree mostly' is the wordplay.
'where' becomes 'thea' (I can't explain this - if you can you should give a lot more credence to this answer).
'mostly' means to remove the last letter (most but not all of the word).
'tree' with its last letter removed is 'tre'.
'thea'+'tre'='THEATRE'
Can you help me to learn more?
(Other definitions for theatre that I've seen before include "stage productions" , "Building for drama" , "(Lecture) room" , "Building for the dramatic arts" , "Where you might find ham" .)