A small enclosure for small tree (5)
I believe the answer is:
aspen
'tree' is the definition.
(aspen is a kind of tree)
'a small enclosure for small' is the wordplay.
'small enclosure' becomes 'pen' (I've seen this before).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'small' becomes 's' (abbreviation - e.g. clothes size).
'pen' after 's' is 'spen'.
'a'+'spen'='ASPEN'
(Other definitions for aspen that I've seen before include "Colorado ski resort - tree" , "city in Colorado" , "Colorado winter sports town" , "Tree, kind of poplar" , "Panes (anag.)" .)