… their old money, trading capital for a billion workers (8)

I believe the answer is:

builders

'workers' is the definition.
The answer and definition can be both people as well as being plural nouns.
Perhaps you can see a link between them that I don't see?

I can't explain the remainder of the clue.

(Other definitions for builders that I've seen before include "Construction workers" , "Constructors" .)

I've seen this clue in The Guardian.

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