Part of a joule (3)
I believe the answer is:
erg
'part of a joule' is the definition.
('erg' is part of 'joule')
This is the entire clue.
(Other definitions for erg that I've seen before include "Amount of work" , "Unit of work or energy" , "Shifting dunes in Sahara" , "Unit of measurement of work" , "Unit of work; dunes area" .)