Studio assistant beginning to eat beef (5)
I believe the answer is:
gripe
'beef' is the definition.
(I know that beef can be written as gripe)
'studio assistant beginning to eat' is the wordplay.
'studio assistant' becomes 'grip' (I've seen this before).
'beginning to' says to take the initial letters.
The initial letter of 'eat' is 'e'.
'grip'+'e'='GRIPE'
(Other definitions for gripe that I've seen before include "Grumble (colloq.)" , "carp" , "Grievance" , "Protest" , "Complain, grouse" .)