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