Brilliant people run away (3)
I believe the answer is:
ace
'brilliant' is the definition.
(ace can informally mean very good)
'people run away' is the wordplay.
'people' becomes 'race' (people can mean a national group or race).
'run' becomes 'r' (cricket abbreviation).
'away' indicates named letters should be taken away.
'race' with 'r' taken out is 'ACE'.
(Other definitions for ace that I've seen before include "Top playing card" , "reason for point of no return" , "Person who excels, a top card" , "1" , "hotshot" .)