Within a month be replaced by six who could lay down law in Rome? (8)
I believe the answer is:
decemvir
'who could lay down law in rome?' is the definition.
(type of Roman magistrate)
'within a month be replaced by six' is the wordplay.
'within' means one lot of letters goes inside another.
'a month' becomes 'Dec' (abbreviation for December).
'be replaced by' becomes 'emr' (I am not sure about this - if you are sure you should believe this answer much more).
'six' becomes 'VI' (Roman numerals).
'dec'+'emr'='decemr'
'decemr' going around 'vi' is 'DECEMVIR'.
(Another definition for decemvir that I've seen is " One of a group of ten Roman lawmakers".)