Investigator's regret about supporting special relationship (10)
I believe the answer is:
rapporteur
'investigator's' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I can't see?
'regret about supporting special relationship' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'about' says the letters should be written backwards.
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'special relationship' becomes 'rapport' (I've seen this before. I am not sure about the 'special' bit.**).
'rue' backwards is 'eur'.
'eur' put after 'rapport' is 'RAPPORTEUR'.
Can you help me to learn more?
(Other definitions for rapporteur that I've seen before include "researcher" , "Record-keeper" , "One investigating" , "Recorder" .)