Type of poplar tree in a small enclosure (5)
I believe the answer is:
aspen
'type of poplar tree' is the definition.
'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'
'in' is the link.
(Other definitions for aspen that I've seen before include "Tree, kind of poplar" , "ski resort in America?" , "somewhere in Colorado" , "Growth" , "Tmorous" .)