### Clue: At once I disposed of poison (7)
### Answer: ACONITE 
 ### Explanation: Definition: poison
  **Anagram of**(disposed of) AT ONCE I.
Answer:  A poison derived from plants of the genus Aconitum.  Highly toxic, but also used after processing or in very small doses in Indian and Chinese medicine.  And some are grown as ornamental garden plants!  A nice surface.