Antiseptic substance (6)
I believe the answer is:
iodine
'antiseptic substance' 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 "Solution of a halogen in ethyl alcohol used on wounds as an antiseptic" , "Infamous tablets in case of a nuclear emergency" , "Halogen element, atomic no 53" , "Antiseptic made from seaweed" , "Dark-violet antiseptic" .)