Tree in a small enclosure (5)
I believe the answer is:
'tree' is the definition.
(aspen is a kind of tree)
'a small enclosure' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'enclosure' becomes 'pen' (pen is a kind of enclosure).
'in' is the link.
(Other definitions for aspen that I've seen before include "The tree that shivers in the slightest wind" , "Tremulous poplar" , "quill-like?" , "Tree, kind of poplar" , "somewhere in Colorado" .)