This is one tough conundrum, for instance (6)

I believe the answer is:

puzzle

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'this is one tough conundrum for instance' is the definition.
The definition suggests a singular noun which matches the answer.

This is the entire clue.

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(Other definitions for puzzle that I've seen before include "Game that tests knowledge or ingenuity, this one say" , "See 24" , "See 13" , "Ingenious problem" , "Perplex, bewilder" .)

I've seen this clue in the Irish Times.

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