Ruling possessed by medic, thoughtful (5)
I believe the answer is:
edict
'ruling' is the definition.
(I have seen 'Authoritative ruling ' mean 'edict' so perhaps 'ruling' could also mean 'edict')
'possessed by medic thoughtful' is the wordplay.
'possessed by' says the answer is hidden in the clue.
'EDICT' is hidden in the letters of 'medic thoughtful'.
(Other definitions for edict that I've seen before include "Formal announcement" , "Legally binding proclamation" , "Authoritative decree" , "Official decree" , "Cited (anag) - proclamation" .)