Vehicle covered by bill sticker — that's promotion! (11)
I believe the answer is:
advancement
'promotion' is the definition.
('advancement' can be a synonym of 'promotion')
'vehicle covered by bill sticker' is the wordplay.
'vehicle' becomes 'van' (van is a kind of vehicle).
'covered by' means one lot of letters go next to another (on top of in a down clue).
'bill' becomes 'ad' (advertisement stuck on wall).
'sticker' becomes 'cement' (something which 'sticks').
'van' after 'ad' is 'advan'.
'advan'+'cement'='ADVANCEMENT'
'that's' is the link.
(Other definitions for advancement that I've seen before include "headway" , "Process of developing" , "making progress" , "Promotion in rank" , "getting on" .)