Crossword Genius

Politicians' union (9)

Ross

I believe the answer is:

coalition

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'union' is the definition.
(I know that coalition is a type of union)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for coalition that I've seen before include "Alliance of parties" , "form of government" , "Temporary alliance" .)

I've seen this clue in the Evening Standard.
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