Crossword Genius

Great distance, around 100 kilometres (6)

Ross

I believe the answer is:

mickle

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'great distance around 100 kilometres' is the definition.
I can't tell whether this definition defines the answer.

'distance around 100 kilometres' is the wordplay.
'distance' becomes 'mile' (mile is a unit of distance).
'around' indicates putting letters inside.
'100' becomes 'C' (Roman numeral).
'kilometres' becomes 'k'.
'c'+'k'='ck'
'mile' placed around 'ck' is 'MICKLE'.

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I've seen this clue in The Times.
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