Crossword Genius

The one woman every political party should have (5)

Ross

I believe the answer is:

unity

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'the one' is the definition.
('unity' can be a synonym of 'one')

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for unity that I've seen before include "A single whole" , "State of being joined as a whole" , "Oneness, harmony" , "Accord" , "Mitford girl" .)

I've seen this clue in the Evening Standard.
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