A county you known has a treatment centre (4)
I believe the answer is:
kent
'a county' is the definition.
(I know that Kent is a county)
'known has a treatment centre' is the wordplay.
'known' becomes 'ket' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'has' indicates putting letters inside.
'a treatment' becomes 'attention' (I have seen 'special treatment' mean 'attention' so perhaps 'treatment' could also mean 'attention').
'centre' says to take the centre.
The middle letter of 'attention' is 'n'.
'ket' enclosing 'n' is 'KENT'.
'you' is the link.
Can you help me to learn more?
(Other definitions for kent that I've seen before include "Home county" , "Lear's faithful Earl" , "Part of 1 25 [KING LEAR]" , "South-eastern county" , "Southern English county" .)