For auditors, generate an invoice in the form of a flyer (9)
I believe the answer is:
razorbill
'flyer' is the definition.
(type of bird)
'for auditors generate an invoice in the form of a' is the wordplay.
I cannot quite see how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is located in the answer.
'form' could be 'orb' (orb is a kind of form) and 'orb' is found in the answer.
'a' is within the answer.
This explanation may well be incorrect...
(Other definitions for razorbill that I've seen before include "Bird" , "Type of auk" , "flier" .)