It is in the tax (5)
I believe the answer is:
tithe
'tax' is the definition.
(historical tax paid to the church)
'it is in the' is the wordplay.
'is in' means one lot of letters goes inside another.
'it' placed inside 'the' is 'TITHE'.
(Other definitions for tithe that I've seen before include "Tax paid to church" , "Tenth (of one's earnings)" , "It used to decimate income" , "Tenth part paid to a church" , "donation to church" .)