Line used in smear campaign (3)
I believe the answer is:
arc
'line' is the definition.
(arc is a kind of line)
'in smear campaign' is the wordplay.
'in' indicates a hidden word.
'ARC' is hidden in the letters of 'smear campaign'.
'used' is the link.
(Other definitions for arc that I've seen before include "Curved path" , "Semi-circle" , "Electrical spark" , "Section of circumference" , "Light" .)