Part of brief is calculation of public revenues (6)
I believe the answer is:
'of public revenues' is the definition.
(I've seen this before)
'part of brief is calculation' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'FISCAL' is hidden within 'brief is calculation'.
(Other definitions for fiscal that I've seen before include "Relating to the public revenue" , "Of public revenues" , "Such a drag" , "In respect of tax revenues" , "Relating to tax" .)