Beer of kings served up (5)
I believe the answer is:
lager
'beer' is the definition.
(type of beer)
'of kings served up' is the wordplay.
'of kings' becomes 'regal' (regal means majestic or fit for royals).
'served up' shows that the letters should be reversed in order (in a down clue, going upwards).
'regal' in reverse letter order is 'LAGER'.
(Other definitions for lager that I've seen before include "A regal sort of drink" , "light ale" , "A brew" , "Light-bodied beer" , "Light beer needed" .)