Whistler really experiencing freedom, first of all (3)
I believe the answer is:
ref
'whistler' is the definition.
(I've seen this before)
'really experiencing freedom first of all' is the wordplay.
'first of all' indicates taking the first letters.
The initial letters of 'really experiencing freedom' is 'REF'.
(Other definitions for ref that I've seen before include "Football's whistler" , "In brief, a soccer controller" , "Football official (colloq.)" , "One traditionally in black" , "A diminutive umpire in soccer or boxing" .)