Beer's served up in error (4)
I believe the answer is:
slip
'error' is the definition.
(slip is a kind of error)
'beer's served up' is the wordplay.
'beer' becomes 'pils'.
'served up' is a reversal indicator.
'pils' back-to-front is 'SLIP'.
'in' is the link.
(Other definitions for slip that I've seen before include "Lingerie item - fielding position" , "printer's proof" , "Mistake (of tongue or pen)" , "One on cricket field" , "Error; watered clay" .)