Sulphur, key preservative (4)
I believe the answer is:
salt
'preservative' is the definition.
(I've seen this in another clue)
'sulphur key' is the wordplay.
'sulphur' becomes 'S' (synonyms).
'key' becomes 'alt' (the Alt keyboard button).
's'+'alt'='SALT'
(Other definitions for salt that I've seen before include "Mariner; sodium chloride" , "Condiment - experienced sailor" , "A defence against ice" , "Seasoning; old sailor" , "dry wit" .)