Half area ignored by Liberal Party, in effect (5,3,4)

I believe the answer is:

fifty per cent

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

'half' is the definition.
(I've seen this before)

'area ignored by liberal party in effect' is the wordplay.
I cannot quite see how this works, but
'by' could be 'per' and 'per' is located in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Another definition for fifty per cent that I've seen is " Half".)

I've seen this clue in The Guardian.

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