Clue: Lithium put in a double sulphate to make garlic, say (6)
Answer: ALLIUM
Explanation: Definition: garlic, say
Heavy on the science. An envelope ('put in') of LI (chemical symbol, 'lithium') in ALUM (potassium aluminium sulphate, or other 'double sulphate'), for the genus which includes 'garlic', leek and onion.