Reflective royal's drink (5)
I believe the answer is:
lager
'drink' is the definition.
(lager is a kind of drink)
'reflective royal's' is the wordplay.
'reflective' says the letters should be written in reverse.
'royal' becomes 'regal' ('regal' can be a synonym of 'royal').
'regal' reversed gives 'LAGER'.
(Other definitions for lager that I've seen before include "Drink - 5 down (anag)" , "Light-bodied beer" , "Regal kind of beer" , "Bottom-fermented beer" , "A brew" .)