One day in seven I'll eradicate ragwort, primarily with this? (10)

'this?' is the definition.
The definition suggests a singular noun which matches the answer.

'one day in seven i'll eradicate ragwort primarily' is the wordplay.
'one day' becomes 'd' (abbreviation for day).
'in' indicates putting letters inside.
'seven' becomes 'week' (I am not sure about this - if you are sure you should believe this answer much more).
'primarily' indicates taking the first letters.
The first letters of 'eradicate ragwort' is 'er'.
'd' inserted within 'week' is 'weedk'.
'weedk'+'ill'+'er'='WEEDKILLER'

'with' is the link.

Can you help me to learn more?