Drunk knocked back small drink (4)
I believe the answer is:
toss
'drink' is the definition.
(as in tossing a drink back)
'drunk knocked back small' is the wordplay.
'drunk' becomes 'sot' (synonyms).
'knocked back' says the letters should be written backwards.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sot' back-to-front is 'tos'.
'tos'+'s'='TOSS'
(Other definitions for toss that I've seen before include "Do it with caber in Scotland" , "Throw (coin) so as to choose" , "Raise a bit of money before opening" , "Spin of a coin" , "Throw lightly and casually" .)