Up with Ben in the mill, messing with weight tied to a string (9)

'up with ben in the mill messing' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'weight tied to a string' is the wordplay.
I cannot quite understand how this works, but
'weight' could be 'plumb' (plumb is a kind of weight) and 'plumb' is present in the answer.
'string' could be 'line' (line is a kind of string) and 'line' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

'with' is the link.

Can you help me to learn more?