Drink is knocked back, first of pints (3)
I believe the answer is:
sip
'drink' is the definition.
(sipping is a kind of drinking)
'is knocked back first of pints' is the wordplay.
'knocked back' says the letters should be written in reverse.
'first of' says to take the initial letters.
The first letter of 'pints' is 'p'.
'is' written backwards gives 'si'.
'si'+'p'='SIP'
(Other definitions for sip that I've seen before include "Drink gingerly" , "Take small mouthfuls of (drink)" , "Drink, small mouthful" , "Drink daintily" , "Drink slowly" .)