Line used by car company (3)
I believe the answer is:
arc
'line' is the definition.
(I know that arc is a type of curve)
'used by car company' is the wordplay.
'used by' indicates a hidden word.
'ARC' is hidden in the letters of 'car company'.
(Other definitions for arc that I've seen before include "Section of circumference" , "Section of curve" , "A curve; a discharge" , "Electrical spark" , "Part of circle; discharge" .)