Employees, terribly fast and furious initially (5)
I believe the answer is:
staff
'employees' is the definition.
(I've seen this in another clue)
'terribly fast and furious initially' is the wordplay.
'terribly' indicates anagramming the letters.
'and' says to put letters next to each other.
'initially' indicates taking the first letters.
The initial letter of 'furious' is 'f'.
'fast' with letters rearranged gives 'staf'.
'staf'+'f'='STAFF'
(Other definitions for staff that I've seen before include "Administrators etc - stick" , "Walking-stick" , "Body of teachers" , "teachers employed in school" , "the general's assistants recognised" .)