Tenth part (5)
I believe the answer is:
tithe
'tenth part' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tithe that I've seen before include "Tenth (of one's earnings)" , "Former church tax" , "Levy of one tenth of something" , "Source of church revenue" , "One-tenth; church tax" .)