It’s naughty, yes, and a bit nice (3)
I believe the answer is:
sin
'a bit nice' is the definition.
I can't tell whether this definition defines the answer.
'it's naughty yes and' is the wordplay.
'it's naughty yes' becomes 'si' (I can't justify this - if you can you should give a lot more credence to this answer).
'and' becomes 'n' (common abbreviation for 'and').
'si'+'n'='SIN'
Can you help me to learn more?
(Other definitions for sin that I've seen before include "Moral offence" , "Immoral action" , "Pride, say" , "Wickedness" , "Trespass" .)