Fiat's ad-men carpeted regularly (6)
I believe the answer is:
decree
'fiat's' is the definition.
('decree' can be a synonym of 'fiat')
'ad-men carpeted regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'ad'+'men'+'carpeted'='admencarpeted'
The alternate letters of 'admencarpeted' are 'DECREE'.
(Other definitions for decree that I've seen before include "Edict; court order" , "Official order of ruling" , "Recede following an official order" , "Official edict" , "Judgment of a court" .)