Splendid drink knocked back (5)
I believe the answer is:
regal
'splendid' is the definition.
(I've seen this in another clue)
'drink knocked back' is the wordplay.
'drink' becomes 'lager' (lager is a kind of drink).
'knocked back' says the letters should be written backwards.
'lager' reversed gives 'REGAL'.
(Other definitions for regal that I've seen before include "Grand and imposing" , "King-like" , "Royal - 5 down (anag)" , "Fit for a king" , "Kingly or queenly" .)