One very loudly boarding transport upset cars? (7)
I believe the answer is:
traffic
'cars?' is the definition.
'one very loudly boarding transport upset' is the wordplay.
'one' becomes 'i' (Roman numeral).
'very loudly' becomes 'ff'.
'boarding' means one lot of letters goes inside another.
'transport' becomes 'cart' (cart is a kind of transport**).
'upset' shows that the letters should be reversed in order.
'i'+'ff'='iff'
'iff' going inside 'cart' is 'ciffart'.
'ciffart' backwards is 'TRAFFIC'.
(Other definitions for traffic that I've seen before include "wheels?" , "Flow of vehicles" , "Business" , "Passing vehicles - trade" , "Smuggle" .)