Serious revolutionary Einstein rejected an imaginary number (7)

I believe the answer is:

intense

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'serious' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for intense that I've seen before include "Excited" , "Acute, vivid" , "Very marked" , "Powerful" , "In an extreme degree" .)

I've seen this clue in The Irish Times.

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