Sinner, sick, to cash in chips (6)
I believe the answer is:
baddie
'sinner' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I can't see?
'sick to cash in chips' is the wordplay.
'sick' becomes 'bad' (similar in meaning).
'to cash in chips' becomes 'die' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bad'+'die'='BADDIE'
Can you help me to learn more?
(Other definitions for baddie that I've seen before include "Villain in film or story" , "Child's name for a villain" , "law breaker?" , "Nasty person" , "Villain (slang)" .)