Knock back beer fit for a monarch (5)
I believe the answer is:
regal
'fit for a monarch' is the definition.
(I've seen this before)
'knock back beer' is the wordplay.
'knock back' says the letters should be written backwards.
'beer' becomes 'lager' (lager is a kind of beer).
'lager' backwards is 'REGAL'.
(Other definitions for regal that I've seen before include "Royal - 5 down (anag)" , "Showing great dignity" , "befitting a king" , "Magnificent, dignified" , "Like a queen?" .)