Cunning old monarch, a Sagittarius (6)
I believe the answer is:
archer
'a sagittarius' is the definition.
(I've seen this in another clue)
'cunning old monarch' is the wordplay.
'cunning' becomes 'arch' (arch can mean cunning or tricksy).
'old monarch' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
'arch'+'er'='ARCHER'
(Other definitions for archer that I've seen before include "Jeffrey --, writer" , "stars" , "User of bows and arrows" , "Bow and arrow user" , "launcher of projectile" .)