4 down
River
trade
subtracting
tax
at
the
end
(5)
River trade subtracting tax at the end (5)
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The definition is at the beginning of the clue.
How the clue works
The definition is River. Think of INDUSTRY, meaning trade, then lop TRY off the end — TRY doing the work of "tax" in the sense of strain or burden — and you're left with your answer.