King’s up for a drink (5)
I believe the answer is:
lager
'for a drink' is the definition.
(lager is a kind of drink)
'king's up' is the wordplay.
'king's' becomes 'regal' (I've seen this before).
'up' is a reversal indicator (in down clue: letters go upwards).
'regal' in reverse letter order is 'LAGER'.
(Other definitions for lager that I've seen before include "Continental beer" , "Pilsner, for example" , "The drink could be regal" , "Local offering" , "Lout's drink?" .)