Witches were once such a topic (7,5)

I believe the answer is:

burning issue

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'witches were' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'such a topic' is the wordplay.
'such' becomes 'burning' (I can't justify this - if you can you should believe this answer much more).
'a topic' becomes 'issue' ('issue' can be a synonym of 'topic').
'burning'+'issue'='BURNING ISSUE'

'once' acts as a link.

Can you help me to learn more?

(Another definition for burning issue that I've seen is " Urgent problem".)

I've seen this clue in the Evening Standard.

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