Some relief is calculated, subject to revenue (6)
I believe the answer is:
fiscal
'to revenue' is the definition.
(I've seen this before)
'some relief is calculated subject' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'FISCAL' is hidden within 'relief is calculated subject'.
(Other definitions for fiscal that I've seen before include "Involving financial matters" , "Of public finances" , "Related to taxes" , "In respect of tax revenues" , "Financial; to do with taxes" .)