It could kill, though not down as a poison (4)
I believe the answer is:
upas
'a poison' is the definition.
(I've seen this before)
'it could kill though not down as' is the wordplay.
I cannot quite understand how this works, but
'as' is within the answer.
'not down' could be 'up' ('up' is the opposite to 'down') and 'up' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for upas that I've seen before include "Sap used to poison arrows" , "A poisoner" , "Antiar tree of Java" , "Java tree providing arrow poison" , "Source of poison" .)