Drop of beer knocked back (4)
I believe the answer is:
slip
'drop' is the definition.
(I've seen this before)
'beer knocked back' is the wordplay.
'beer' becomes 'pils'.
'knocked back' shows that the letters should be reversed in order.
'pils' written backwards gives 'SLIP'.
'of' acts as a link.
(Other definitions for slip that I've seen before include "Lingerie item - fielding position" , "Fall or make a small mistake" , "Silly little mistake" , "printer's proof" , "Error; lose one's footing" .)