Nark catching American dons teaching English the second I expressed it's unnecessary (14)

'nark' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I can't see?

'american dons teaching english the second i expressed it's unnecessary' is the wordplay.
I cannot quite understand how this works, but
'american' could be 'us' (United States) and 'us' is found within the answer.
'english' could be 'e' (abbreviation) and 'e' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'second' could be 's' (common abbreviation) and 's' is located in the answer.
'i' could be 'a' and 'a' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
This explanation may well be incorrect...

'catching' acts as a link.

Can you help me to learn more?