Watt-hour fraction (3)
I believe the answer is:
erg
'watt-hour fraction' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for erg that I've seen before include "Unit of measurement of work" , "What's used to record work" , "Unit of work or energy" , "shifting sand" , "Unit of work; dunes area" .)