Snake in French tree (5)
I believe the answer is:
aspen
'tree' is the definition.
(aspen is a kind of tree)
'snake in french' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'in french' becomes 'en' ('in' in French).
'asp'+'en'='ASPEN'
(Other definitions for aspen that I've seen before include "Trembling poplar tree" , "Tmorous" , "quill-like?" , "Colorado ski resort - tree" , "Tree with shivering leaves" .)