Hard to restrict parking by a huge amount (7)

I believe the answer is:

steeply

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'by a huge amount' is the definition.
I can't tell whether this defines the answer.

'hard to restrict parking' is the wordplay.
'hard' becomes 'steely' (similar in meaning).
'to restrict' means one lot of letters goes inside another.
'parking' becomes 'p' (abbreviation on road signs).
'steely' placed around 'p' is 'STEEPLY'.

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I've seen this clue in The Times and The Sunday Times.

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