Where are the billboards? Zero adverts seen during journey (8)
I believe the answer is:
roadside
'where are the billboards?' is the definition.
(billboards might be found at the roadside)
'zero adverts seen during journey' is the wordplay.
'zero' becomes 'O' (O resembles 0).
'adverts' becomes 'ads' ('ad' can be a synonym of 'advert').
'seen during' is an insertion indicator.
'journey' becomes 'ride' (riding is a kind of journeying).
'o'+'ads'='oads'
'oads' inserted into 'ride' is 'ROADSIDE'.
(Other definitions for roadside that I've seen before include "Bordering on the street" , "by the way" , "Edge of the highway" , "kerb" , "Carriageway border" .)