Element with symbol I (6)
I believe the answer is:
iodine
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">element with symbol i</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)