It is included in the donation (5)
I believe the answer is:
tithe
'donation' is the definition.
(tithe is a kind of donation)
'it is included in the' is the wordplay.
'is included in' indicates putting letters inside.
'it' placed inside 'the' is 'TITHE'.
(Other definitions for tithe that I've seen before include "Tax to support the church" , "Tenth (of one's earnings)" , "One-tenth of income paid to church" , "Church payment" , "Tax paid to church" .)