Snake with space on tree (5)
I believe the answer is:
aspen
'tree' is the definition.
(aspen is a kind of tree)
'snake with space' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'with' means one lot of letters go next to another.
'space' becomes 'en'.
'asp'+'en'='ASPEN'
'on' acts as a link.
(Other definitions for aspen that I've seen before include "winter resort" , "Trembler" , "Panes (anag.)" , "Trembling poplar tree" , "city in Colorado" .)