Knocking back lager is a slight mistake (4)
I believe the answer is:
slip
'a slight mistake' is the definition.
(I've seen this before)
'knocking back lager' is the wordplay.
'knocking back' is a reversal indicator.
'lager' becomes 'pils' (I've seen this before).
'pils' written backwards gives 'SLIP'.
'is' acts as a link.
(Other definitions for slip that I've seen before include "printer's proof" , "Cutting" , "Man in field" , "Drop" , "failure" .)