Support medical treatment that hurts (6)
I believe the answer is:
pillow
'support' is the definition.
(thesaurus)
'medical treatment that hurts' is the wordplay.
'medical treatment' becomes 'pill'.
'that hurts' becomes 'ow!'.
'pill'+'ow'='PILLOW'
(Other definitions for pillow that I've seen before include "Soft place for the head" , "Head supporter in bed" , "Bolster, perhaps" , "Sleeper's support" , "Bed cushion" .)