Official with a whistle (3)
I believe the answer is:
ref
'official with a whistle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ref that I've seen before include "Whistleblower (abbr)" , "'Game judge, in short (3)'" , "Football's whistler" , "One traditionally in black" , "In brief, he controls a soccer match" .)