Splendid Belgrade resident goes round on horseback
I believe the answer is:
superb
'splendid' is the definition.
(I've seen this in another clue)
'belgrade resident goes round on horseback' is the wordplay.
'belgrade resident' becomes 'serb' (I've seen this in another clue).
'goes round' indicates putting letters inside.
'on horseback' becomes 'up' (up can mean up on a horse).
'serb' enclosing 'up' is 'SUPERB'.
(Other definitions for superb that I've seen before include "Really excellent, brilliant" , "Outstandingly excellent" , "Magnificent, quite splendid" , "Splendid, brilliant" , "choice" .)