Union formed by producers of togas? (5)

I believe the answer is:

sogat

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'producers of togas?' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'union formed by' is the wordplay.
I cannot really understand how this works, but
'by' could be 'at' and 'at' is found in the answer.
The remaining letters 'sog' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

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I've seen this clue in the Evening Standard.

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