Whistle-blower in Hereford (3)
I believe the answer is:
ref
'whistle-blower' is the definition.
(referees blow whistles)
'in hereford' is the wordplay.
'in' indicates a hidden word.
'REF' can be found hidden inside 'hereford'.
(Other definitions for ref that I've seen before include "Football official (colloq.)" , "Whistleblower (abbr)" , "A diminutive umpire in soccer or boxing" , "One traditionally in black" , "In brief, a soccer controller" .)