Even in sorrow taxpayers contribute to it (7)
I believe the answer is:
revenue
'taxpayers contribute to it' is the definition.
I can't judge whether this defines the answer.
'even in sorrow' is the wordplay.
'in' means one lot of letters goes inside another.
'sorrow' becomes 'rue' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'even' put within 'rue' is 'REVENUE'.
Can you help me to learn more?
(Other definitions for revenue that I've seen before include "Money coming in" , "made?" , "stream, perhaps" , "A country's income as from taxes" , "Ready" .)