Knocked back royal beer (5)
I believe the answer is:
lager
'beer' is the definition.
(lager is a kind of beer)
'knocked back royal' is the wordplay.
'knocked back' shows that the letters should be reversed in order.
'royal' becomes 'regal' ('regal' can be a synonym of 'royal').
'regal' written backwards gives 'LAGER'.
(Other definitions for lager that I've seen before include "A regal sort of drink" , "Pilsner, for example" , "light ale" , "Regal sort of beer" , "Regal kind of beer" .)