They are sent by letter to demand payment (4-10)

'they are sent by letter' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'to demand payment' is the wordplay.
I cannot quite see how this works, but
'to' is present in the answer.
'payment' could be 'rent' (rent is a kind of payment) and 'rent' is located in the answer.
This may be the basis of clue (or it may be nonsense).