Work unit (3)
'work unit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for day that I've seen before include "THIRD PART OF 23" , "Particular date" , "24 hour period" , "Unit of time" , "Dawn to dusk" .)
'work unit' is the definition.
(I know that erg is a type of work unit)
This is the entire clue.
(Other definitions for erg that I've seen before include "Amount of work" , "What's used to record work" , "Energy expended" , "Unit of work; dunes area" , "Shifting dunes in Sahara" .)