Diminutive whistler (3)
I believe the answer is:
ref
This clue might be a double definition.
'diminutive' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'whistler' is the second definition.
(a referee blows a whistle)
Can you help me to learn more?
(Other definitions for ref that I've seen before include "In brief, he controls a soccer match" , "Middleman needing cards" , "A diminutive umpire in soccer or boxing" , "'Game judge, in short (3)'" , "Sports official (abbrev.)" .)