Maybe black marks, thanks to being caught in wrongdoing (6)
I believe the answer is:
stains
'maybe black marks' is the definition.
'stains' can be an answer for 'marks' ('stain' can be a synonym of 'mark'). I am unsure of the 'maybe black' bit.
'thanks to being caught in wrongdoing' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'to being caught in' means one lot of letters goes inside another.
'wrongdoing' becomes 'sins' (I can't justify this - if you can you should believe this answer much more).
'ta' placed within 'sins' is 'STAINS'.
Can you help me to learn more?
(Other definitions for stains that I've seen before include "Discolourations on satins" , "Sullies" , "Dirty marks" , "Discolorations or blemishes" , "Discolorations on satins" .)