Tree found in a small enclosure (5)
I believe the answer is:
aspen
'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).
'a'+'s'+'pen'='ASPEN'
'found in' is the link.
(Other definitions for aspen that I've seen before include "Trembler" , "ski resort in America?" , "Tree that quivers" , "Its leaves flutter in the slightest breeze" , "Tmorous" .)