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E,
for
Einstein?
(7)
E, for Einstein? (7)
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The whole clue is the definition (& lit or double definition).
How the clue works
The whole clue is a whimsical E, for Einstein? definition, punning on EGGHEAD (a genius like Einstein). The letter E is literally the "egg head" — the head letter of the word EGG — and Einstein is a classic egghead, so the two ideas merge into one cryptic definition.