Millionth of a metre (6)

I believe the answer is:

micron

'millionth of a metre' is the definition.
(I've seen this before)

This is all the clue.

(Other definitions for micron that I've seen before include "One-millionth of a metre" , "One millionth of 1 metre" , "One millionth of 3 Across" , "Micrometre" , "Small length" .)

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

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