I, on the periodic table (6)
I believe the answer is:
iodine
'i on the periodic table' 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" , "Antiseptic made from seaweed" , "Hanging tuft" , "Element that can exist as a purple vapour" , "Element forming black crystals or violet vapour" .)