Protest about breaking ruler (7)
I believe the answer is:
monarch
'ruler' is the definition.
(I know that monarch is a type of ruler)
'protest about breaking' is the wordplay.
I cannot quite understand how this works, but
'about' could be 'on' (as in a book 'on' a certain topic) and 'on' is found in the answer.
'protest' could be 'march' (marching is a kind of protesting) and 'march' is present in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for monarch that I've seen before include "Stag in Landseer picture" , "Eg, king, queen, emperor" , "Head of State" , "Not a subject" , "Sole hereditary ruler" .)