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"persistent" is also used as a crossword clue. If that's what you saw, the answer is often dogged (6 letters).
Definitions
Continuing firmly or for a long time despite difficulty.
Word origin
From Latin persistentem, present participle of persistō (“continue steadfastly”). By surface analysis, persist + -ent.
Used in a sentence
“She has had a persistent cough for weeks.”
“There was a persistent knocking on the door.”
“There have been persistent rumours for years.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer2 curated clues
01“Continuing firmly”10 letters
02“Hard to stop”10 letters
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