Departments blocking ex-prime minister's supporters in former industrial areas (10,5)

I believe the answer is:

brownfield sites

I'm a little stuck... Click here to teach me more about this clue!

'departments blocking ex-prime minister's supporters in former industrial' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'areas' is the wordplay.
I cannot really understand how this works, but
'areas' could be 'fields' (field is a kind of area) and 'fields' is located in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Independent.

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