Fraction of a watt-hour (3)
I believe the answer is:
erg
'fraction of a watt-hour' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for erg that I've seen before include "Unit of work or energy" , "What's used to record work" , "Amount of work" , "Energy expended" , "Unit of work (phys.)" .)