Tips to help all dodging fifty per cent in tax (5)
I believe the answer is:
tithe
'tax' is the definition.
(historical tax paid to the church)
I don't understand how the remainder of the clue works.
(Other definitions for tithe that I've seen before include "Tenth part (as tribute)" , "Tenth part - old tax" , "Levy of one tenth of something" , "Ancient tax" , "Levy by church, e.g." .)