"Stephen's tongue", is what we utter as monarch rules (5,7)
I believe the answer is:
kings english
'stephen's' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?
'tongue is what we utter as monarch rules' is the wordplay.
I cannot really see how this works, but
'tongue' could be 'english' (English is an example) and 'english' is found in the answer.
'as' could be 'in' (I've seen this in other clues) and 'in' is found within the answer.
'monarch' could be 'k' (abbreviation for king in chess) and 'k' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for kings english that I've seen before include "Correct way to speak, pre-1952?" , "'Correct' language of the largest part of 1?" .)