Possible Clues:

• One day in seven I'll eradicate ragwort, primarily with this?
• Sound like Doc gets a job as an executioner getting rid of unwanted intruders in the plot
• Used by 14 across to get rid of intruders with pot that could be the death of you
• Gets rid of intruders from the plot offering local drugs that can be fatal
• Gardener's aid against unwanted intruders
• Substance used to get rid of unwanted plants