One who's always doing the wrong thing (6)
I believe the answer is:
sinner
'one who's always doing' is the definition.
I can't tell whether this definition defines the answer.
'wrong thing' is the wordplay.
'wrong' becomes 'sin' (wrong can mean a sin or crime).
'thing' becomes 'ner' (I can't justify this - if you can you should give a lot more credence to this answer).
'sin'+'ner'='SINNER'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for sinner that I've seen before include "One in breach of moral law" , "Immoral transgressor" , "Transgressor of the moral law" , "One who breaches the moral law" , "Offender against moral law" .)