Ump's relative (3)
I believe the answer is:
ref
'ump's relative' 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 "A diminutive umpire in soccer or boxing" , "Whistleblower (abbr)" , "Football's whistler" , "In brief, a soccer controller" , "One traditionally in black" .)