Drink is knocked back quietly (3)
I believe the answer is:
sip
'drink' is the definition.
(sipping is a kind of drinking)
'is knocked back quietly' is the wordplay.
'knocked back' says the letters should be written in reverse.
'quietly' becomes 'p' (p for piano and quiet).
'is' in reverse letter order is 'si'.
'si'+'p'='SIP'
(Other definitions for sip that I've seen before include "Small mouthful of fluid" , "Smallest drink" , "Drink slowly" , "Thimbleful" , "Drink, small mouthful" .)