Treatment costs nothing, so unconcerned (8)
I believe the answer is:
carefree
'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).
'care'+'free'='CAREFREE'
'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" .)