Puzzle imagined to be tricky — not the first or last (6)

I believe the answer is:

enigma

'puzzle' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for enigma that I've seen before include "Coding device" , "Puzzle of gamine (6)," , "Riddle - German coding machine" , "Conundrum" , "Mysterious puzzle" .)

I've seen this clue in The Guardian.

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