Sn, to a chemist (3)
I believe the answer is:
tin
'sn to a chemist' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tin that I've seen before include "Silvery-white metallic element" , "Light metal" , "A silvery-white metal, symbol Sn" , "Metal element" , "1 down, symbol Sn" .)