Such a lark was apt to be dirty! (3)
I believe the answer is:
mud
'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" .)