In place of an enclosure, a tree (5)
I believe the answer is:
aspen
'a tree' is the definition.
(aspen is a kind of tree)
'in place of an enclosure' is the wordplay.
'in place of' becomes 'as' (I can't justify this - if you can you should believe this answer much more).
'an enclosure' becomes 'pen' (pen is a kind of enclosure).
'as'+'pen'='ASPEN'
Can you help me to learn more?
(Other definitions for aspen that I've seen before include "Trembling poplar tree" , "Tmorous" , "Very timid" , "Colorado winter sports town" , "Trembler" .)