Whistle-blower hiding up in safe room (3)
I believe the answer is:
ref
'whistle-blower' is the definition.
(referees blow whistles)
'hiding up in safe room' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'hiding' indicates the answer is hidden within the clue.
'fer' is hidden within 'in safe room'.
'fer' written backwards gives 'REF'.
(Other definitions for ref that I've seen before include "In brief, he controls a soccer match" , "One traditionally in black" , "Sports umpire" , "Whistleblower (abbr)" , "Football official (colloq.)" .)