Joule fraction (3)
I believe the answer is:
erg
'joule 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 "Shifting dunes in Sahara" , "shifting sand" , "Unit of work or energy" , "Energy expended" , "What's used to record work" .)