Drink's knocked back, linger for piece of cake (8)
I believe the answer is:
'piece of cake' is the definition.
('pushover' can be a synonym of 'piece of cake')
'drink's knocked back linger' is the wordplay.
'drink' becomes 'sup' (I've seen this before).
'knocked back' shows that the letters should be reversed in order.
'linger' becomes 'hover' ('hover' can be a synonym of 'linger').
'sup' written backwards gives 'pus'.
'for' is the link.
(Other definitions for pushover that I've seen before include "One is easily defeated or outwitted" , "A thing easily done" , "upset" , "Person easily convinced or deceived - or shoved down" , "breeze" .)