What does the cryptic crossword clue "Drink, knock it back as a starter (4)" mean?

The definition is "Drink,". The wordplay uses an anagram. anagram of IT + AS = ASTI; definition: "Drink," The answer is ASTI.

Drink, knock it back as a starter (4)

Guardian Cryptic #22048 | 9 Across | 2000-11-06 flower