Professor's most important fundamental truth (9)

I believe the answer is:

principle

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

I don't understand how the rest of the clue works.

(Other definitions for principle that I've seen before include "General scientific law" , "In which, theoretically" , "General guiding rule" , "Precept, rule" , "honour" .)

I've seen this clue in The Independent.

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