Briefly a judge? (3)
I believe the answer is:
ref
'a judge' is the definition.
(referee)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ref that I've seen before include "Whistleblower (abbr)" , "In brief, a soccer controller" , "'Game judge, in short (3)'" , "Football official (colloq.)" , "One traditionally in black" .)