Brought out one’s briefcase initially in error (4)
I believe the answer is:
boob
'error' is the definition.
(thesaurus)
'brought out one's briefcase initially' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'brought out ones briefcase' is 'BOOB'.
'in' acts as a link.
(Other definitions for boob that I've seen before include "Blunder (colloq.)" , "Error" , "Slip" , "Mistake; breast (sl.)" , "Embarrassing mistake" .)