Millionth of a metre (6)
I believe the answer is:
'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" .)