Reportedly strikes base outside large US city (9)

'city' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I don't see?

'reportedly strikes base outside large us' is the wordplay.
'reportedly' shows a homophone (sound like).
'strikes' becomes 'knocks' (knocking is a kind of striking).
'base' becomes 'vile' ('vile' can be a synonym of 'base').
'outside' indicates putting letters inside.
'large us' becomes 'l' (I can't explain this - if you can you should believe this answer much more).
'knocks' sounds like 'knox'.
'vile' placed around 'l' is 'ville'.
'knox'+'ville'='KNOXVILLE'