Knocking back lager a mistake? (4)
I believe the answer is:
slip
'a mistake?' is the definition.
(slip is a kind of mistake)
'knocking back lager' is the wordplay.
'knocking back' is a reversal indicator.
'lager' becomes 'pils' (Pilsner beer).
'pils' reversed gives 'SLIP'.
(Other definitions for slip that I've seen before include "Fall on the thinned potter's clay" , "release > voucher" , "Silly little mistake" , "Error; lose one's footing" , "Minor mistake - undergarment" .)