Some regret slurs over province (6)
I believe the answer is:
ulster
'province' is the definition.
(I've seen this before)
'some regret slurs over' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'over' says the letters should be written in reverse.
'retslu' is hidden within 'regret slurs'.
'retslu' reversed gives 'ULSTER'.
(Other definitions for ulster that I've seen before include "former province of Ireland" , "Area of Ireland" , "Man's overcoat" , "Cloak for an Irish province" , "Overcoat; Irish region" .)