Carefree attitude a nice cousin cultivated (11)
I believe the answer is:
insouciance
'carefree attitude' is the definition.
(insouciance is indifference or nonchalance)
'a nice cousin cultivated' is the wordplay.
'cultivated' is an anagram indicator.
'a'+'nice'+'cousin'='anicecousin'
'anicecousin' is an anagram of 'INSOUCIANCE'.
(Other definitions for insouciance that I've seen before include "Casual unconcern" , "Lack of concern" .)