Unit of energy (3)

I believe the answer is:

erg

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

This is all the clue.

(Other definitions for erg that I've seen before include "shifting sand" , "Unit of measurement of work" , "Energy expended" , "What's used to record work" , "Unit of work or energy" .)

I've seen this clue in the Evening Standard, the LA Times, the Penny Press and The Guardian.

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