The thing comes in for tax (5)
I believe the answer is:
tithe
'tax' is the definition.
(I've seen this before)
'the thing comes in' is the wordplay.
'thing' becomes 'it'.
'comes in' means one lot of letters goes inside another.
'the' enclosing 'it' is 'TITHE'.
'for' acts as a link.
(Other definitions for tithe that I've seen before include "donation to church" , "Ancient tax" , "Tenth (of one's earnings)" , "that was given to clergyman" , "One-tenth of income paid to church" .)