A wretch from Northern Ireland turned one down (7)
I believe the answer is:
ingrate
'a wretch' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'northern ireland turned one down' is the wordplay.
'northern ireland' becomes 'NI' (abbreviation).
'turned' says the letters should be written in reverse.
'one down' becomes 'grate' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ni' written backwards gives 'in'.
'in'+'grate'='INGRATE'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for ingrate that I've seen before include "Thankless person from Tangier" , "Thankless wretch" , "Unthankful person" , "Unappreciative character" , "Thankless type" .)