It is included in the tax (5)
I believe the answer is:
tithe
'tax' is the definition.
(historical tax paid to the church)
'it is included in the' is the wordplay.
'is included in' is an insertion indicator.
'it' put inside 'the' is 'TITHE'.
(Other definitions for tithe that I've seen before include "One-tenth of income paid to church" , "It used to decimate income" , "Impost" , "donation to church" , "Levy of one tenth of something" .)