Tree snake at the bottom of the garden (5)
I believe the answer is:
aspen
'tree' is the definition.
(aspen is a kind of tree)
'snake at the bottom of the garden' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'at the bottom of' suggests the final letters (I've seen 'at the bottom' mean this).
The final letters of 'the garden' are 'en'.
'asp'+'en'='ASPEN'
(Other definitions for aspen that I've seen before include "Colorado ski resort - tree" , "One trembling" , "Trembler" , "Tmorous" , "winter resort" .)