It's extra for the solicitor (5)

I believe the answer is:

rider

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

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for rider that I've seen before include "One may be saddled" , "Condition added to something already agreed" , "Traveller on horseback" , "extra weight needed for balance?" , "Contract clause" .)

I've seen this clue in the Evening Standard.

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