Business's ending belly up, getting black mark (4)
I believe the answer is:
smut
'black mark' is the definition.
'smut' can be an answer for 'mark' (thesaurus). I am unsure of the 'black' bit.
'business's ending belly up' is the wordplay.
'ending' indicates one should take the final letters (the ending of the word).
'belly' becomes 'tum' (I've seen this before).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
The last letter of 'business' is 's'.
'tum' written backwards gives 'mut'.
's'+'mut'='SMUT'
'getting' is the link.
Can you help me to learn more?
(Other definitions for smut that I've seen before include "Pornography or grime" , "Dirt or pornography" , "Dirty object or talk" , "Pornographic material" , "Dirty stuff" .)