Fermi, for one (4)
I believe the answer is:
unit
'fermi for one' is the definition.
(fermi is a kind of unit)
This is all the clue.
(Other definitions for unit that I've seen before include "Any division of quantity accepted as a standard of measurement" , "Single person - group of people" , "Piece of furniture" , "- - trust" , "Single element; one" .)