Talk idly about regular letters in biggest puzzle (6)

I believe the answer is:

jigsaw

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'puzzle' is the definition.
(jigsaw puzzle is a type of puzzle)

I don't understand the remainder of the clue.

(Other definitions for jigsaw that I've seen before include "Puzzle in which pieces must be reassembled" , "As pastime" , "Puzzle with pieces to assemble" , "Puzzle fittingly completed" , "Woodcutter" .)

I've seen this clue in The Times.

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