Ruler about to stop military display (7)
I believe the answer is:
monarch
'ruler' is the definition.
(I know that queen can be written as monarch)
'about to stop military display' is the wordplay.
'about' becomes 'on' (as in a book 'on' a certain topic).
'to stop' indicates putting letters inside.
'military display' becomes 'march' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' placed inside 'march' is 'MONARCH'.
Can you help me to learn more?
(Other definitions for monarch that I've seen before include "Eg, king, queen, emperor" , "Crowned head" , "one over realm?" , "Butterfly - ruler" , "Not a subject" .)