Medical centre in Gwent's treatment for pain (6)
I believe the answer is:
twinge
'for pain' is the definition.
(twinge is a kind of pain)
'medical centre in gwent's treatment' is the wordplay.
'centre' says to take the centre.
'in' indicates putting letters inside.
'treatment' indicates anagramming the letters (letters treated or processed differently).
The centre of 'medical' is 'i'.
'gwent' is an anagram of 'twnge'.
'i' inserted inside 'twnge' is 'TWINGE'.
(Other definitions for twinge that I've seen before include "Brief sharp pain" , "Sudden localised pain" , "Sudden sharp feeling" , "Qualm" , "Momentary pain" .)