It's in the church's interest (5)
I believe the answer is:
tithe
'church's interest' is the definition.
The answer and definition can be both related to property as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'it's in the' is the wordplay.
'in' indicates putting letters inside.
'it' put into 'the' is 'TITHE'.
Can you help me to learn more?
(Other definitions for tithe that I've seen before include "Tenth part once paid as tax" , "It used to decimate income" , "church funding" , "Tax to support the church" , "Tenth part (as tribute)" .)