Beer knocked over in error (4)
I believe the answer is:
slip
'error' is the definition.
(I know that slip is a type of error)
'beer knocked over' is the wordplay.
'beer' becomes 'pils' (Pilsner beer).
'knocked over' says the letters should be written in reverse.
'pils' reversed gives 'SLIP'.
'in' is the link.
(Other definitions for slip that I've seen before include "He should take his chances" , "old coin" , "Fail < to engage gear" , "Slide; petticoat" , "Careless mistake" .)