Crossword Genius

Several books in one large vehicle (7)

Ross

I believe the answer is:

omnibus

I'm a little stuck... Click here to teach me more about this clue!

This clue is a quadruple definition!!

'several books' is the first definition.
(I've seen this before)

'in' is a link.

'one' is the second definition.
(thesaurus)

'large' is the third definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?

'vehicle' is the fourth definition.
(I've seen this before)

Can you help me to learn more?

(Other definitions for omnibus that I've seen before include "public service" , "Large public vehicle" , "Compendium volume" , "edition of book" , "one conveying passengers" .)

I've seen this clue in The Telegraph.
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