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Thwart (6)

Ross

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7 9

(Other definitions for derail that I've seen before include "Cause to run off the tracks" , "Cause to come off the tracks" , "go off track" , "create an obstructive diversion" , "go off line" .)

I've seen this clue in The Independent, the King Feature Syndicate, the LA Times, The Guardian, The Sunday Times, The Sun, the Evening Standard and The Telegraph.
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