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

'nark' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see an association 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 in the answer.
'english' could be 'e' (abbreviation) and 'e' is located 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 present in the answer.
'i' could be 'a' and 'a' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
This explanation may well be incorrect...

'catching' acts as a link.

Can you help me to learn more?