FBI agent accepted regulation was imperfect (6)
I believe the answer is:
flawed
'imperfect' is the definition.
(similar in meaning)
'fbi agent accepted regulation' is the wordplay.
'fbi agent' becomes 'fed' (short for federal agent).
'accepted' means one lot of letters goes inside another.
'regulation' becomes 'law' (I've seen this in another clue).
'fed' enclosing 'law' is 'FLAWED'.
'was' is the link.
(Other definitions for flawed that I've seen before include "with a hole in it" , "wrong" , "With imperfections" , "Containing blemish or imperfection" , "Defective" .)