Invoice between lecturer and postgrad? (3,5)

I believe the answer is:

pro forma

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'invoice' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for pro forma that I've seen before include "Document to be filled in" , "Standard form" , "Invoice" , "in a prescribed manner" , "In a set manner, a formality only" .)

I've seen this clue in The Independent.

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