One parting hurt such a relationship (7)
I believe the answer is:
married
'one' is the definition.
(thesaurus)
'parting hurt such a relationship' is the wordplay.
'parting' means one lot of letters goes inside another (inserted letters split or part another word).
'hurt' becomes 'marred' (mar can mean to hurt or damage).
'such a relationship' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'marred' placed around 'i' is 'MARRIED'.
Can you help me to learn more?
(Other definitions for married that I've seen before include "Admirer (anag)" , "United, wed" , "Wedded" , "9 across (anag)" , "Hitched" .)