Splendid beer knocked back (5)
I believe the answer is:
regal
'splendid' is the definition.
(thesaurus)
'beer knocked back' is the wordplay.
'beer' becomes 'lager' (lager is a kind of beer).
'knocked back' is a reversal indicator.
'lager' back-to-front is 'REGAL'.
(Other definitions for regal that I've seen before include "fit for the Queen" , "Imperial, majestic" , "Royal - 5 down (anag)" , "King's" , "Showing great dignity" .)