Horse trading group? (3)

I believe the answer is:

set

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'group?' is the definition.
(both can mean a collection)

'horse trading' is the wordplay.
'SET' is hidden within the letters.
But, I am uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for set that I've seen before include "Class" , "Mount (precious stone)" , "Fix in place; similar group" , "exclusive party" , "Group; hard" .)

I've seen this clue in the Evening Standard.

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