Base; unit (4)
I believe the answer is:
foot
This clue is a double definition.
'base' is the first definition.
(I know that base can be written as foot)
'unit' is the second definition.
(I know that foot is an Unit of length)
(Other definitions for foot that I've seen before include "Distance to the end of a limb?" , "Labour Party leader, 1980-83" , "support" , "Measure - part of body" , "Metrical unit; pay (a bill)" .)