E for Einstein? (6)

I believe the answer is:

energy

'e for einstein?' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for energy that I've seen before include "Vitality, power" , "Dynamism" , "Healthy capacity for vigorous activity" , "Zest" , "Power; capacity for work" .)

I've seen this clue in The Sun and The Telegraph.

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