Slo-mo reviewer (3)
I believe the answer is:
ref
'slo-mo reviewer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ref that I've seen before include "Sports official (abbrev.)" , "'Game judge, in short (3)'" , "Whistleblower (abbr)" , "One traditionally in black" , "In brief, he controls a soccer match" .)