Help when presenting US car line on the radio (7)
I believe the answer is:
autocue
'help when presenting' is the definition.
(device telling a TV presenter what to say)
'us car line on the radio' is the wordplay.
'us car' becomes 'auto' (US term for a car).
'line' becomes 'queue' (queue is a kind of line).
'on the radio' indicates a 'sounds like' (homophone) clue.
'queue' sounds like 'cue'.
'auto'+'cue'='AUTOCUE'
(Other definitions for autocue that I've seen before include "Speaker's prompt device" , "speaking aid" , "Prompting device for a speaker" , "TV signal" , "Device displaying words to be read" .)