Line used in car compound (3)
I believe the answer is:
arc
'line' is the definition.
(arc is a kind of line)
'in car compound' is the wordplay.
'in' says the answer is hidden in the clue.
'ARC' is hidden in the letters of 'car compound'.
'used' is the link.
(Other definitions for arc that I've seen before include "A curve for Joan of . . ." , "bends?" , "Part of curved line" , "Section of circumference" , "Something used to weld" .)