It's about the tax (5)
I believe the answer is:
tithe
'tax' is the definition.
(thesaurus)
'it's about the' is the wordplay.
'it's' becomes 'it' (I've seen this in other clues).
'about' is a reversal indicator.
'it' back-to-front is 'ti'.
'ti'+'the'='TITHE'
(Other definitions for tithe that I've seen before include "It used to decimate income" , "property tax" , "Tenth (of one's earnings)" , "Gift of tenth" , "Source of church revenue" .)