Unit of work in the lower grades (3)
I believe the answer is:
erg
'unit of work' is the definition.
(I've seen this in another clue)
'in the lower grades' is the wordplay.
'in' indicates a hidden word.
'ERG' can be found hidden inside 'lower grades'.
(Other definitions for erg that I've seen before include "Unit of work or energy" , "Energy expended" , "What's used to record work" , "shifting sand" , "Unit of work; dunes area" .)