Crossword Genius

Promise a choice between seven days and one? (4)

Ross

I believe the answer is:

word

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'promise' is the definition.
(as in 'I give you my word')

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for word that I've seen before include "Promise - unit of language" , "Message" , "See 21" , "See 19" , "Brief statement" .)

I've seen this clue in The Telegraph.
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