Douglas, imitated by William? (4)

I believe the answer is:

haig

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'douglas' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I can't see?

I don't understand the rest of the clue.

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(Other definitions for haig that I've seen before include "Earl -; British WW 1 commander-in-chief" , "WWI C-in-C" , "Douglas -, field marshal" , "British World War One field marshal" .)

I've seen this clue in the Evening Standard.

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