Crossword Genius

Part of shoe Alf's found in cavity (4-4)

Ross

I believe the answer is:

half-sole

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'part of shoe' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'alf's found in cavity' is the wordplay.
'found in' is an insertion indicator.
'cavity' becomes 'hole' (hole is a kind of cavity).
'alfs' put within 'hole' is 'HALF-SOLE'.

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I've seen this clue in The Times.
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