New aim to limit danger, mostly, in ruler's domains (7)
I believe the answer is:
imperia
'new aim' is the definition.
The definition suggests a singular noun which matches the answer.
'to limit danger mostly in ruler's domains' is the wordplay.
'to limit' indicates putting letters inside (inserted letters are restrained or limited).
'danger' becomes 'peril' (peril is a kind of danger).
'mostly' means to remove the last letter (most but not all of the word).
'in ruler's domains' becomes 'ima' (I am not sure about this - if you are sure you should believe this answer much more).
'peril' with its final letter removed is 'peri'.
'peri' put into 'ima' is 'IMPERIA'.
Can you help me to learn more?