I, in chemistry (6)
I believe the answer is:
iodine
'i in chemistry' 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 "Nonmetallic element in seaweed" , "Tincture for cuts" , "Antiseptic found in seaweed" , "Element existing in vapour and crystalline form" , "Halogen element, atomic no 53" .)