It's put in the kind of barns churches once found helpful (5)
I believe the answer is:
tithe
'kind of barns churches once found helpful' is the definition.
(as in tithe barns)
'it's put in the' is the wordplay.
'put in' is an insertion indicator.
'it' placed within 'the' is 'TITHE'.
(Other definitions for tithe that I've seen before include "Tenth part paid to a church" , "property tax" , "that was given to clergyman" , "One tenth - old church tax" , "Tenth of income given to church" .)