Knock back lager? That's a mistake (4)
I believe the answer is:
slip
'that's a mistake' is the definition.
(a slip is a blunder or mistake)
'knock back lager?' is the wordplay.
'knock back' is a reversal indicator.
'lager?' becomes 'pils' (Pilsner beer).
'pils' reversed gives 'SLIP'.
(Other definitions for slip that I've seen before include "Strip" , "shift" , "Cricket fielder - mistake" , "Minor mistake - undergarment" , "pre-printed form" .)