Knock the beer back: easily one’s undoing? (8)
I believe the answer is:
slipknot
'easily one's undoing?' is the definition.
(knot that can be easily undone)
'knock the beer back' is the wordplay.
'knock' becomes 'tonk' (tonk can mean to hit hard).
'the beer' becomes 'pils' (Pilsner beer).
'back' shows that the letters should be reversed in order.
'tonk'+'pils'='tonkpils'
'tonkpils' backwards is 'SLIPKNOT'.
(Other definitions for slipknot that I've seen before include "With which noose is tied" , "connector - with 12a" , "Something tied but easily undone" .)