A limit first on urban areas' extents (10)

'extents' is the definition.
The answer and definition can be both human attributes as well as being plural nouns.
Maybe you can see an association between them that I don't see?

'a limit first on urban areas'' is the wordplay.
'limit first' becomes 'cap' (capping is a kind of limiting. I am not sure about the 'first' bit.).
'on' means one lot of letters go next to another.
'urban areas'' becomes 'cities'' (city is a kind of urban area).
'a' put after 'cap' is 'capa'.
'capa'+'cities'='CAPACITIES'

Can you help me to learn more?