Growing source of arrow poison (4)
I believe the answer is:
upas
This clue might be a double definition.
'growing source' is the first definition.
'upas' can be an answer for 'source' (I have seen 'Source of poison' mean 'upas' so perhaps 'source' could also mean 'upas'). I am unsure of the 'growing' bit.
'arrow poison' is the second definition.
(I've seen this in another clue)
'of' acts as a link.
Can you help me to learn more?
(Other definitions for upas that I've seen before include "Poisonous Javanese tree" , "Tree from Java with poisonous sap" , "A poisoner" , "Antiar tree of Java" , "Source of poison" .)