The one woman every political party should have (5)
I believe the answer is:
unity
'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" .)