Treatment costs nothing, so unconcerned (8)
I believe the answer is:
'unconcerned' is the definition.
(similar in meaning)
'treatment costs nothing' is the wordplay.
'treatment' becomes 'care' (I've seen this in another clue).
'costs nothing' becomes 'free' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'so' is the link.
Can you help me to learn more?
(Other definitions for carefree that I've seen before include "Without anxiety or responsibilities" , "Unconstrained" , "Jaunty" , "Untroubled" , "Happy and blithe - no worries" .)