The poor fellow can't do any good (6)
I believe the answer is:
sinner
'the poor fellow' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sinner that I've seen before include "Immoral transgressor" , "One in breach of moral law" , "Wrong-doer" , "One morally corrupt" , "Evildoer" .)