Book transport for a lot of people (7)
I believe the answer is:
'book' is the definition.
(I know that omnibus is a type of book)
'transport for a lot of people' is the wordplay.
'transport' becomes 'bus' (busing is a kind of transporting).
'for' means one lot of letters go next to another (I've seen this in other clues).
'a lot of people' becomes 'omni' (I can't explain this - if you can you should give a lot more credence to this answer).
'bus' put after 'omni' is 'OMNIBUS'.
Can you help me to learn more?
(Other definitions for omnibus that I've seen before include "edition of book" , "Volume containing several books" , "A number of books in one" , "collection of novels?" , "Passenger-carrier" .)