It is central to this major urban district (5-4)
I believe the answer is:
inner-city
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'central to this major urban district' is the wordplay.
I cannot really understand how this works, but
'central' could be 'inner' (similar in meaning) and 'inner' is found within the answer.
'urban' could be 'city' and 'city' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Another definition for inner-city that I've seen is " Very built-up area".)