Very cool beer drained and knocked back is bitter (5)

I believe the answer is:

acerb

I'm a little stuck... Click here to teach me more about this clue!

'is bitter' is the definition.
(synonyms)

I don't understand how the remainder of the clue works.

(Other definitions for acerb that I've seen before include "Sour, bitter" , "harsh and bitter" , "tart" , "sharp" .)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.