Champion in the rotunda (4)
I believe the answer is:
hero
'champion' is the definition.
('hero' can be a synonym of 'champion')
'in the rotunda' is the wordplay.
'in' indicates the answer is hidden within the clue.
'HERO' can be found hidden inside 'the rotunda'.
(Other definitions for hero that I've seen before include "girl a ring" , "Man admired for courage and achievement" , "One looked up" , "lover whose lamp went out tragically" , "Star" .)