Crossword Genius

Such a lark was apt to be dirty! (3)

Ross

I believe the answer is:

mud

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'dirty' is the definition.
(mudding is a kind of dirtying)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for mud that I've seen before include "Liquid earth" , "Here's . . . in your eye - it's a toast" , "Earth and water mixed" , "damaging allegations" , "Wet sticky earth" .)

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