Salt additive (6)
I believe the answer is:
iodine
'salt additive' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for iodine that I've seen before include "Element, atomic number 53" , "Mild antiseptic" , "Mother-of-pearl" , "Element with symbol I" , "Hanging tuft" .)