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