Agreement concerning some country estate? (3)
I believe the answer is:
yes
'agreement' is the definition.
(saying yes to something)
'some country estate?' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'YES' is hidden within 'country estate'.
'concerning' is the link.
(Other definitions for yes that I've seen before include "Indeed" , "Affirmative answer" , "That's 27 [RIGHT]" , "3 across" , "Pro vote" .)