Drink fit for a king knocked back (5)
I believe the answer is:
lager
'drink' is the definition.
(lager is a kind of drink)
'fit for a king knocked back' is the wordplay.
'fit for a king' becomes 'regal' (I've seen this before).
'knocked back' says the letters should be written in reverse.
'regal' reversed gives 'LAGER'.
(Other definitions for lager that I've seen before include "Light beer needed" , "The drink could be regal" , "Pilsner, for example" , "Regal kind of beer" , "Light-bodied beer" .)