"Tree snake" in French (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' (French word for 'in').
'asp'+'en'='ASPEN'
(Other definitions for aspen that I've seen before include "Tmorous" , "winter resort" , "Tree, kind of poplar" , "Tree with shivering leaves" , "Trembler" .)