Acacia pith beyond gross plant with poisonous sap (5)
I believe the answer is:
sumac
'plant with poisonous sap' is the definition.
'sumac' can be an answer for 'plant' (I've seen this before). I am not sure about the 'with poisonous sap' bit.
'acacia pith beyond gross' is the wordplay.
I cannot quite understand how this works, but
'pith' could be 'sum' (synonyms) and 'sum' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sumac that I've seen before include "Smoke-bush" , "Small tree bright in autumn" , "plant" , "Tree colourful in autumn" , "Garden shrub" .)