Physics unit (3)
I believe the answer is:
erg
'physics unit' 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 "What's used to record work" , "Shifting dunes in Sahara" , "Unit of work or energy" , "Unit of work (phys.)" , "Energy expended" .)