Drink royal upset (5)
I believe the answer is:
lager
'drink' is the definition.
(lager is a kind of drink)
'royal upset' is the wordplay.
'royal' becomes 'regal' ('regal' can be a synonym of 'royal').
'upset' says the letters should be written in reverse.
'regal' back-to-front is 'LAGER'.
(Other definitions for lager that I've seen before include "A regal sort of drink" , "Light beer; sounds like camp" , "Light beer needed" , "Effervescent beer" , "Regal (anag) - drink" .)