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"challenging" is also used as a crossword clue. If that's what you saw, the answer is often hard (4 letters).
Definitions
requiring full use of your abilities or resources
Word origin
From Middle English chalengynge; equivalent to challenge + -ing.
Used in a sentence
“There are always sincere challengings of the findings, always the objections (sincere in another sense) of those whose interests seem threatened.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer1 curated clues
01“Hard to do”11 letters
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