Go over every monarch's disloyalty (9)
I believe the answer is:
treachery
'disloyalty' is the definition.
(treachery is a kind of disloyalty)
'go over every monarch's' is the wordplay.
'go' becomes 'try' (I've seen this before).
'over' indicates putting letters inside.
'every' becomes 'each' ('each' can be a synonym of 'every').
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'each'+'er'='eacher'
'try' going around 'eacher' is 'TREACHERY'.
(Other definitions for treachery that I've seen before include "disloyalty" , "Double-dealing" , "perfidious conduct" , "Perfidy" , "Treason" .)