Key to a vacant lot (3)

I believe the answer is:

alt

'key' is the definition.
(the Alt keyboard button)

'a vacant lot' is the wordplay.
'vacant' means to remove the middle letters (a vacant or emptied out form of the word).
'lot' with its middle taken out is 'lt'.
'a'+'lt'='ALT'

'to' acts as a link.

I've seen this clue in the Sydney Morning Herald.

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