The most common and most likely answer to this crossword clue is the 3 letter word ERG
I've seen this crossword clue in Independent
E
R
G
Buddy explains!
The straight part
We know that "quantity of work" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case quantity of work = erg.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ERG mean?
ERG noun- The unit of work or energy, being the amount of work done by a force of one dyne applied through a distance of one centimeter. Equal to 10−7 joules.
- A large desert region of sand dunes with little or no vegetation, especially in the Sahara.
- An ergometer.
- To use an ergometer.