Crossword Genius

Commander-in-chief of the British forces in France during the First World War (4)

Ross

I believe the answer is:

haig

I'm a little stuck... Click here to teach me more about this clue!

'world war' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's an association between them I don't understand?

'commander-in-chief of the british forces in france during the first' is the wordplay.
I cannot quite understand how this works, but
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is found within the answer.
The remaining letters 'hag' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

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

I've seen this clue in The Times.
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