Aunt's dog in wild really impressive (10)
I believe the answer is:
astounding
'really impressive' is the definition.
(astounding means very impressive or surprising)
'aunt's dog in wild' is the wordplay.
'wild' indicates anagramming the letters.
'aunts'+'dog'+'in'='auntsdogin'
'auntsdogin' is an anagram of 'ASTOUNDING'.
(Other definitions for astounding that I've seen before include "Extraordinary" , "Amazing" , "Stunning" .)