In serious pain, one might go there for treatment (3)
I believe the answer is:
spa
'might go there for treatment' is the definition.
I can't tell whether this defines the answer.
'in serious pain one' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SPA' can be found hidden inside 'serious pain one'.
Can you help me to learn more?
(Other definitions for spa that I've seen before include "well" , "place of treatment" , "Watering place" , "Curative resort" , "E.g. Harrogate" .)