Norfolk's secondary highways? (6)

I believe the answer is:

broads

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'norfolk's secondary highways?' is the definition.
I can't tell whether this definition defines the answer.

'secondary highways?' is the wordplay.
'secondary' becomes 'b' (I can't justify this - if you can you should believe this answer much more).
'highways?' becomes 'roads' (highway is a kind of road).
'b'+'roads'='BROADS'

Can you help me to learn more?

(Other definitions for broads that I've seen before include "Norfolk holiday area" , "Old coins" , "found in part of eastern England" , "Wetland" .)

I've seen this clue in the Evening Standard.

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