Knock back drink and linger for piece of cake (8)
I believe the answer is:
pushover
'piece of cake' is the definition.
(synonyms)
'knock back drink and linger' is the wordplay.
'knock back' says the letters should be written in reverse.
'drink' becomes 'sup' (to sup is to drink).
'and' is a charade indicator (letters next to each other).
'linger' becomes 'hover' (synonyms).
'sup' written backwards gives 'pus'.
'pus'+'hover'='PUSHOVER'
'for' is the link.
(Other definitions for pushover that I've seen before include "One is easily defeated or outwitted" , "picnic" , "breeze" , "upset" , "Child's play, piece of cake" .)