It was in the church's interest (5)
I believe the answer is:
tithe
'church's interest' is the definition.
The definition and answer can be both related to property as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'it was in the' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'was in' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
't'+'i'+'the'='TITHE'
Can you help me to learn more?
(Other definitions for tithe that I've seen before include "10 per cent levy" , "Regular payments" , "financial contribution" , "Ancient tax" , "Tenth of income given to church" .)