Doing things wrong, one is termed 'poor' (6)
I believe the answer is:
sinner
'doing' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'wrong one is termed poor' is the wordplay.
'wrong' becomes 'sin' (wrong can mean a sin or crime).
'one is' becomes 'ne' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'termed' indicates one should take the final letters (I've seen 'term of' mean this (term can mean the terminus or ending of the word)).
The last letter of 'poor' is 'r'.
'sin'+'ne'+'r'='SINNER'
'things' acts as a link.
Can you help me to learn more?
(Other definitions for sinner that I've seen before include "Wrong-doer" , "Serious offender" , "Offender against moral law" , "One in error" , "One in breach of moral law" .)