Element with symbol I (6)
I believe the answer is:
iodine
'element with symbol i' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for iodine that I've seen before include "Infamous tablets in case of a nuclear emergency" , "Element existing in vapour and crystalline form" , "for therapeutic purposes" , "Antiseptic found in seaweed" , "Element forming black crystals or violet vapour" .)