Prompt car to wait in line, audibly (7)
I believe the answer is:
autocue
'prompt' is the definition.
'car to wait in line audibly' is the wordplay.
'car' becomes 'auto' ('auto' can be a synonym of 'car').
'to wait in line' becomes 'queue' (I've seen this before).
'audibly' shows a homophone (sound like).
'queue' sounds like 'cue'.
'auto'+'cue'='AUTOCUE'
(Other definitions for autocue that I've seen before include "Vocal prompter" , "TV presenting aid" , "TV prompter" , "TV text reader" , "TV signal" .)