It divides the fixed-rate tax (5)
I believe the answer is:
tithe
'tax' is the definition.
(historical tax paid to the church)
'it divides the' is the wordplay.
'divides' is an insertion indicator (inserted letters divide the word).
'it' placed into 'the' is 'TITHE'.
'fixed-rate' acts as a link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tithe that I've seen before include "Tenth (of one's earnings)" , "Former church tax" , "Levy, originally worth one tenth" , "Tenth of income given to church" , "property tax" .)