Declared an FBI agent protects business (8)
I believe the answer is:
affirmed
'declared' is the definition.
(affirming is a kind of declaring)
'an fbi agent protects business' is the wordplay.
'an' becomes 'a'.
'fbi agent' becomes 'fed' (short for federal agent).
'protects' means one lot of letters goes inside another (outer letters protect inside letters).
'business' becomes 'firm' (firm is a kind of business).
'fed' going around 'firm' is 'ffirmed'.
'a'+'ffirmed'='AFFIRMED'
(Other definitions for affirmed that I've seen before include "Stated positively and firmly" , "Declared solemnly to be true" , "Endorsed" , "Averred" , "swore" .)