Clue: Invoice of a sort for working poor farm (3,5)
Answer: PRO FORMA
Explanation: Definition: Invoice of a sort
An anagram ('working') of 'poor farm'. *Pro forma*, as a matter of form, may be applied to various things, including an invoice, which would generally be one sent as a quotation, before the goods are sent or the work done. So, do we have here an indication by example? I cannot quite decide.