Entry in brief is calculation relating to tax revenues (6)
I believe the answer is:
'relating to tax revenues' is the definition.
'entry in brief is calculation' is the wordplay.
'entry in' says the answer is hidden in the clue (the answer is entered into the clue).
'FISCAL' can be found hidden inside 'brief is calculation'.
(Other definitions for fiscal that I've seen before include "Relating to state revenue" , "Budgetary" , "regarding tax" , "Related to taxes" , "Financial; to do with taxes" .)