Boodle's mark (4)
I believe the answer is:
blot
This clue could be a double definition.
'boodle's' is the first definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'mark' is the second definition.
(I have seen 'bad mark' mean 'blot' so perhaps 'mark' could also mean 'blot')
Can you help me to learn more?
(Other definitions for blot that I've seen before include "Stain (on landscape?)" , "Dry piece of writing" , "Don't do it to your copybook" , "Blemish made with ink" , "Smudge of ink" .)