Wild child's last, behind mad aunt and me (7)
I believe the answer is:
untamed
'wild' is the definition.
(I know that wild can be written as untamed)
'child's last behind mad aunt and me' is the wordplay.
'last' indicates one should take the final letters.
'behind' says to put letters next to each other.
'mad' indicates anagramming the letters.
'and' means one lot of letters go next to another.
The last letter of 'child' is 'd'.
'aunt'+'me'='auntme'
'auntme' is an anagram of 'untame'.
'd' after 'untame' is 'UNTAMED'.
(Other definitions for untamed that I've seen before include "In a natural state, not domesticated" , "Not yet broken in" , "Not domesticated or subdued" , "Wild, not domesticated" .)