Degenerate duke's command after first part of march? (2,8)

I believe the answer is:

go downhill

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But I can't explain it at all! Could you help me understand?

(Other definitions for go downhill that I've seen before include "Skiers will usually" , "Ski" , "flag" , "take part in skiing event" , "deteriorate" .)

I've seen this clue in The Independent.

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