Stop monarch being mad (7)
I believe the answer is:
barking
'mad' is the definition.
(I've seen this before)
'stop monarch' is the wordplay.
'stop' becomes 'bar' (bar can mean to stop or prevent).
'monarch' becomes 'king' (king is a kind of monarch).
'bar'+'king'='BARKING'
'being' acts as a link.
(Other definitions for barking that I've seen before include "Dog may be" , "Mental" , "Crazy - yapping" , "Mad - town forming a London borough with Dagenham" , "up the pole" .)