Thanks to participating in crime, a bad mark (5)
I believe the answer is:
stain
'a bad mark' is the definition.
'stain' can be an answer for 'mark' ('stain' can be a synonym of 'mark'). I'm not certain of the 'a bad' bit.
'thanks to participating in crime' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'to participating in' means one lot of letters goes inside another.
'crime' becomes 'sin'.
'ta' going within 'sin' is 'STAIN'.
Can you help me to learn more?
(Other definitions for stain that I've seen before include "Penetrative dye; mark" , "Blot" , "Colour, dye" , "Black mark for saint" , "Dirty mark; dye" .)