Party introducing rules completely upset monarch once (6)
I believe the answer is:
oswald
'monarch once' is the definition.
(I know that Oswald was a monarch)
'party introducing rules completely upset' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'introducing' means one lot of letters goes inside another (some letters introduced or inserted into others).
'rules' becomes 'laws' (law is a kind of rule).
'completely upset' says the letters should be written in reverse (in a down clue, letters go up).
'do' enclosing 'laws' is 'dlawso'.
'dlawso' backwards is 'OSWALD'.
(Other definitions for oswald that I've seen before include "chap" , "Lee Harvey . . . . . ., assassin of JFK" , "Lee Harvey . . . . . . assassinated President Kennedy" , "King of the Northumbrians, d. 642 - President Kennedy's assassin, d. 1963" , "Man" .)