Erect as a tree (4)
I believe the answer is:
upas
'a tree' is the definition.
(upas is a species of tree)
'erect as' is the wordplay.
'erect' becomes 'up'.
'up'+'as'='UPAS'
(Other definitions for upas that I've seen before include "A poisoner" , "Poisonous Javanese tree" , "Tree from Java with poisonous sap" , "Source of poison" , "Java tree providing arrow poison" .)