Splendid monarch in riotous pubs (6)
I believe the answer is:
superb
'splendid' is the definition.
(both can mean excellent)
'monarch in riotous pubs' is the wordplay.
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'in' is an insertion indicator.
'riotous' indicates an anagram.
'pubs' with letters rearranged gives 'supb'.
'er' placed within 'supb' is 'SUPERB'.
(Other definitions for superb that I've seen before include "Outstandingly excellent" , "Really excellent, brilliant" , "Splendid, brilliant" , "Gorgeous" , "Quite splendid, really excellent" .)