Second-class highway in Norfolk? (5)

I believe the answer is:

broad

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'norfolk?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'second-class highway' is the wordplay.
'second class' becomes 'b'.
'highway' becomes 'road' (highway is a type of road).
'b'+'road'='BROAD'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for broad that I've seen before include "not detailed" , "Spacious - tolerant" , "extensive" , "Covering a wide range" , "East Anglian waterway" .)

I've seen this clue in the Evening Standard.

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