Check about key monarchy (5)
I believe the answer is:
reign
'monarchy' is the definition.
(thesaurus)
'check about key' is the wordplay.
'check' becomes 'rein' (both can mean to control or limit).
'about' means one lot of letters goes inside another.
'key' becomes 'g' (I've seen this in other clues**).
'rein' enclosing 'g' is 'REIGN'.
(Other definitions for reign that I've seen before include "Occupy the throne" , "Hegemony" , "Dominion" , "Niger (anag)" , "Period on the throne" .)