Mechanic at the rear stops to modify Fiat (5)
I believe the answer is:
edict
'fiat' is the definition.
(synonyms)
'mechanic at the rear stops to modify' is the wordplay.
'at the rear' suggests the final letters.
'stops' is an insertion indicator (stopping can mean blocking a gap).
'to modify' becomes 'edit' (I've seen this in another clue).
The last letter of 'mechanic' is 'c'.
'c' inserted within 'edit' is 'EDICT'.
(Other definitions for edict that I've seen before include "Formal order" , "Injunction" , "Regulation" , "Authoritative ruling" , "An official command" .)