Rule not enforced in the main? (3,2,3,4)

'main?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'rule not enforced in the' is the wordplay.
I cannot really understand how this works, but
'rule' could be 'law' (law is a kind of rule) and 'law' is present in the answer.
'not' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?