Crossword Genius

Something nasty afoot: leader leaving alliance (5)

Ross

I believe the answer is:

union

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'alliance' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for union that I've seen before include "Coupling, merger" , "workers' co-operative" , "Workers' group" , "Workers' federation" , "See 22" .)

I've seen this clue in The Sun.
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