19 across
Duke’s
decline
largely
involving
unknown
figure
in
French
scandal
(7)
Duke’s decline largely involving unknown figure in French scandal (7)
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The definition is at the end of the clue.
How the clue works
Right, so the definition's figure in French scandal — that's Alfred Dreyfus, the chap from the Dreyfus affair. Start with D for Duke, then take REFUSE meaning decline, and lop off the last letter as "largely" tells you to, giving REFUS. Now pop Y (that's your unknown, algebra-style) into the middle of REFUS, and bolt the whole lot onto the D at the front — there's your Frenchman.