Partly the antithesis of a tax (5)
I believe the answer is:
tithe
'a tax' is the definition.
(historical tax paid to the church)
'partly the antithesis of' is the wordplay.
'partly' says the answer is hidden in the clue (part of the clue is used).
'TITHE' is hidden within 'antithesis of'.
(Other definitions for tithe that I've seen before include "Gift of tenth" , "Tenth part (as tribute)" , "Levy of one tenth of something" , "Tenth part once paid as tax" , "It used to decimate income" .)