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 #23330 | 21 Across | 2004-12-20 flower