One who cries foul? (3)
I believe the answer is:
ref
'one who cries foul?' 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 "Sports official (abbrev.)" , "Football official (colloq.)" , "In brief, he controls a soccer match" , "Whistleblower (abbr)" , "One traditionally in black" .)