Line used in car campaign (3)
I believe the answer is:
arc
'line' is the definition.
(I know that arc is a type of curve)
'in car campaign' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ARC' is hidden in the letters of 'car campaign'.
'used' is the link.
(Other definitions for arc that I've seen before include "A curve for Joan of . . ." , "Half-moon" , "Something curved in shape" , "Light" , "Curvature" .)