Solitary types intruded upon by fellow city dwellers (9)

'fellow city dwellers' is the definition.
The answer and definition can be both people as well as being plural nouns.
Perhaps there's a link between them I don't understand?

'solitary types intruded upon' is the wordplay.
I cannot really understand how this works, but
'solitary' could be 'one' (the only one) and 'one' is found in the answer.
'upon' could be 'on' (I've seen this in another clue) and 'on' is found in the answer.
This explanation may well be incorrect...

'by' is the link.

Can you help me to learn more?