Domestic maxim of political significance (4,4)
I believe the answer is:
home rule
'political significance' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'domestic maxim' is the wordplay.
'domestic' becomes 'home' (similar in meaning).
'maxim' becomes 'rule' (I am not sure about this - if you are sure you should believe this answer much more).
'home'+'rule'='HOME RULE'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for home rule that I've seen before include "Demand from nationalist" , "Parnell wanted this" , "Political autonomy" , "Self-government in local matters" , "Independence (as for Ireland)" .)