Mystery at heart of agent's slight remark (6)

I believe the answer is:

enigma

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'mystery' is the definition.
(I know that mystery can be written as enigma)

I can't explain the remainder of the clue.

(Other definitions for enigma that I've seen before include "WW2 code" , "Puzzle found in gem, Ian" , "Mysterious or puzzling thing" , "cryptic puzzle!" , "Puzzle for gaming" .)

I've seen this clue in The Irish Times.

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