Unit of work (3)

I believe the answer is:

erg

'unit of work' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for erg that I've seen before include "Unit of measurement of work" , "Energy expended" , "shifting sand" , "Unit of work or energy" , "Unit of work; dunes area" .)

I've seen this clue in The Washington Post Sunday, the Sydney Morning Herald, the King Feature Syndicate, the Penny Press, the LA Times, The New York Times, The Wall Street Journal, The Guardian and The Irish Times.

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