Faulty regulation adopted by US agent (6)
I believe the answer is:
flawed
'faulty' is the definition.
(thesaurus)
'regulation adopted by us agent' is the wordplay.
'regulation' becomes 'law' (I've seen this before).
'adopted by' indicates putting letters inside (I've seen this in other clues).
'us agent' becomes 'fed' (short for a federal agent).
'law' put inside 'fed' is 'FLAWED'.
(Other definitions for flawed that I've seen before include "Containing blemish or imperfection" , "with a hole in it" , "Imperfect" , "Defective" , "Having blemish or defect" .)