Copied one that's entering on foot (1,4)

I believe the answer is:

a pied

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'on foot' is the definition.
Both the answer and definition are adjectives.
Perhaps you can see a link between them that I don't see?

'copied one that's entering' is the wordplay.
'copied' becomes 'aped' ('ape' can be a synonym of 'copy').
'one' becomes 'i' (Roman numeral).
'that's entering' is an insertion indicator.
'aped' placed around 'i' is 'A PIED'.

Can you help me to learn more?

I've seen this clue in The Guardian.

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