Crossword Genius

A case of tradespeople sticking together (5)

Ross

I believe the answer is:

union

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'a case' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps there's an association between them I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for union that I've seen before include "Amalgamation" , "workers' co-operative" , "Coupling, merger" , "Organisation of employees for bargaining purposes" , "Wedding, say" .)

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