Prevents vast reforms overwhelming monarch (6)
I believe the answer is:
averts
'prevents' is the definition.
(averting is a kind of preventing)
'vast reforms overwhelming monarch' is the wordplay.
'reforms' is an anagram indicator.
'overwhelming' means one lot of letters goes inside another.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'vast' with letters rearranged gives 'avts'.
'avts' enclosing 'er' is 'AVERTS'.
(Other definitions for averts that I've seen before include "Heads off, prevents from happening" , "Wards off" , "Turns aside" , "Of eyes or gaze, turns away" , "Turns away or causes to turn away" .)