Volume of city traffic (7)
I believe the answer is:
omnibus
This could be a double definition.
'volume' is the first definition.
(I have seen 'Compendium volume ' mean 'omnibus' so perhaps 'volume' could also mean 'omnibus')
'city traffic' is the second definition.
'omnibus' can be an answer for 'traffic' (I've seen this before). I'm not certain of the 'city' bit.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for omnibus that I've seen before include "Coach" , "PSV" , "Book - public transport" , "Large passenger vehicle - collection of printed works in one volume" , "Collection of related works reprinted in one volume" .)