Drink fit for a king to knock back? (5)
I believe the answer is:
lager
'drink' is the definition.
(lager is a kind of drink)
'fit for a king to knock back?' is the wordplay.
'fit for a king' becomes 'regal' (I've seen this before).
'to knock back?' is a reversal indicator (I've seen 'knock back' mean this).
'regal' backwards is 'LAGER'.
(Other definitions for lager that I've seen before include "Type of beer" , "Light-coloured beer" , "light ale" , "Continental beer" , "Bottom-fermented beer" .)