Sporting whistler? (3)
I believe the answer is:
ref
'whistler?' is the definition.
(a referee blows a whistle)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ref that I've seen before include "Sports umpire" , "Whistleblower (abbr)" , "Football official (colloq.)" , "Middleman needing cards" , "A diminutive umpire in soccer or boxing" .)