Small drink, peg, knocked back (3)
I believe the answer is:
nip
'small drink' is the definition.
(I've seen this before)
'peg knocked back' is the wordplay.
'peg' becomes 'pin' ('pin' can be a synonym of 'peg').
'knocked back' shows that the letters should be reversed in order.
'pin' written backwards gives 'NIP'.
(Other definitions for nip that I've seen before include "Shot" , "little drink" , "Pinch; drink of spirits" , "smart" , "Pinch; bite" .)