Lack of concern involved us in cocaine (11)
I believe the answer is:
insouciance
'lack of concern' is the definition.
(I've seen this before)
'involved us in cocaine' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'us'+'in'+'cocaine'='usincocaine'
'usincocaine' with letters rearranged gives 'INSOUCIANCE'.
(Other definitions for insouciance that I've seen before include "Lack of concern" , "Casual unconcern" .)