Unit of measurement of work or energy (3)
I believe the answer is:
erg
This clue is a double definition.
'unit of measurement of work' is the first definition.
(I've seen this before)
'energy' is the second definition.
(Other definitions for erg that I've seen before include "Shifting dunes in Sahara" , "Unit of work (phys.)" , "shifting sand" , "Unit of work or energy" , "Amount of work" .)