NFL whistle blower (3)
I believe the answer is:
ref
'nfl whistle blower' 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 "'Game judge, in short (3)'" , "Middleman needing cards" , "Sports umpire" , "Sports official (abbrev.)" , "One traditionally in black" .)