1995 novel by Robert Harris about a young mathematician stationed in Bletchley Park (6)

I believe the answer is:

enigma

(Other definitions for enigma that I've seen before include "It's mysterious, difficult to understand" , "Mysterious person" , "WW two Ger. code device; puzzle" , "Mysterious puzzle" , "WW2 code" .)

I've seen this clue in The Independent.

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