Bill's added extra passenger (5)

I believe the answer is:

rider

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This clue might be a double definition.

'bill's' is the first definition.
(thesaurus)

'added extra passenger' is the second definition.
'rider' can be an answer for 'passenger' (synonyms). I'm not sure about the 'added extra' bit.

Can you help me to learn more?

(Other definitions for rider that I've seen before include "Person on horse" , "Gold coin" , "in the dressing room before the show?" , "Contract clause" , "Contractual clause" .)

I've seen this clue in the Sydney Morning Herald.

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