Secondary highways in Norfolk (6)

I believe the answer is:

broads

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

'secondary highways' is the wordplay.
'secondary' becomes 'b' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'highways' becomes 'roads' (I've seen this in another clue).
'b'+'roads'='BROADS'

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.

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