Agonising treatment in unsympathetic country (4,6)
I believe the answer is:
cold turkey
'agonising treatment' is the definition.
(process when giving up drugs)
'unsympathetic country' is the wordplay.
'unsympathetic' becomes 'cold' (I've seen this before).
'country' becomes 'turkey' (Turkey is an example).
'cold'+'turkey'='COLD TURKEY'
'in' is the link.
(Other definitions for cold turkey that I've seen before include "taken in the days after Christmas?" , "Problems of withdrawal" , "result of stopping drugs" , "Depression" , "Cure" .)