Stop knocking back drink in past (5)
I believe the answer is:
belay
'stop' is the definition.
(nautical term meaning stop)
'knocking back drink in past' is the wordplay.
'knocking back' says the letters should be written in reverse.
'drink' becomes 'ale' (ale is a kind of drink).
'in' means one lot of letters goes inside another.
'past' becomes 'by' (as in years gone by).
'ale' back-to-front is 'ela'.
'ela' placed inside 'by' is 'BELAY'.
(Other definitions for belay that I've seen before include "Make fast" , "Stop" , "Turn a rope round something to secure it" .)