A taxing part (5)

I believe the answer is:

tithe

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'part' is the definition.
(I have seen 'Tenth part' mean 'tithe' so perhaps 'part' could also mean 'tithe')

I cannot understand how the remainder of the clue works.

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(Other definitions for tithe that I've seen before include "10% tax" , "Impost" , "Tenth part once paid as tax" , "Charge" , "Tenth part paid to a church" .)

I've seen this clue in the Evening Standard.

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