Tip back mixed drink (8)
I believe the answer is:
cocktail
'mixed drink' is the definition.
(cocktail is a kind of mixed drink)
'tip back' is the wordplay.
'tip' becomes 'cock' (I've seen this before).
'back' becomes 'tail' (tail is a kind of back).
'cock'+'tail'='COCKTAIL'
(Other definitions for cocktail that I've seen before include "Manhattan, for example" , "old-fashioned in America" , "Manhattan, perhaps" , "dish with prawn" , "Mixed beverage" .)