Careless kids — acacia all messed up (13)
I believe the answer is:
lackadaisical
'careless' is the definition.
(I've seen this before)
'kids acacia all messed up' is the wordplay.
'messed up' is an anagram indicator.
'kids'+'acacia'+'all'='kidsacaciaall'
'kidsacaciaall' is an anagram of 'LACKADAISICAL'.
(Other definitions for lackadaisical that I've seen before include "Spiritless" , "Careless and lazy" , "Carelessly lazy" , "Wanting enthusiasm" , "Half-hearted" .)