Removed from bed, like a poisonous tree (4)
I believe the answer is:
'a poisonous tree' is the definition.
(I've seen this in another clue)
'removed from bed like' is the wordplay.
'removed from bed' becomes 'up'.
'like' becomes 'as'.
(Other definitions for upas that I've seen before include "Antiar tree of Java", "Java tree providing arrow poison", "Tree from Java with poisonous sap", "Tree yielding poisonous sap", "Source of poison".)