Fathom or foot (4)
I believe the answer is:
unit
'fathom or foot' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for unit that I've seen before include "Individual component" , "Single element; one" , "Single thing or person" , "Any division of quantity accepted as a standard of measurement" , "Part of a kit" .)