Basic food preservative (4)
I believe the answer is:
salt
'basic food preservative' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for salt that I've seen before include "Common substance" , "White crystalline substance" , "Crystalline substance used to season food" , "Condiment - old sailor" , "Condiment - experienced sailor" .)